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| author | Dmitri Gribenko <gribozavr@gmail.com> | 2012-12-12 17:02:44 +0000 |
|---|---|---|
| committer | Dmitri Gribenko <gribozavr@gmail.com> | 2012-12-12 17:02:44 +0000 |
| commit | b64f020a30427e90a05530f8b20f1bddd1ab9e0a (patch) | |
| tree | 9c60ef5b98080ca4d953d31238d8946656bf69e6 | |
| parent | 44581afacc7afc5e4e6cb8ffa14d337cf54a279f (diff) | |
| download | llvm-b64f020a30427e90a05530f8b20f1bddd1ab9e0a.tar.gz llvm-b64f020a30427e90a05530f8b20f1bddd1ab9e0a.tar.bz2 llvm-b64f020a30427e90a05530f8b20f1bddd1ab9e0a.tar.xz | |
Documentation: convert WritingAnLLVMPass.html to reST.
Patch by Anthony Mykhailenko with small fixes by me.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169992 91177308-0d34-0410-b5e6-96231b3b80d8
| -rw-r--r-- | docs/CMake.rst | 2 | ||||
| -rw-r--r-- | docs/GarbageCollection.rst | 6 | ||||
| -rw-r--r-- | docs/MakefileGuide.rst | 6 | ||||
| -rw-r--r-- | docs/WritingAnLLVMBackend.rst | 4 | ||||
| -rw-r--r-- | docs/WritingAnLLVMPass.html | 1954 | ||||
| -rw-r--r-- | docs/WritingAnLLVMPass.rst | 1439 | ||||
| -rw-r--r-- | docs/subsystems.rst | 7 | ||||
| -rw-r--r-- | docs/userguides.rst | 5 |
8 files changed, 1456 insertions, 1967 deletions
diff --git a/docs/CMake.rst b/docs/CMake.rst index 9bf0d56255..f89578863c 100644 --- a/docs/CMake.rst +++ b/docs/CMake.rst @@ -355,6 +355,8 @@ an equivalent variant of snippet shown above: target_link_libraries(mycompiler ${REQ_LLVM_LIBRARIES}) +.. _cmake-out-of-source-pass: + Developing LLVM pass out of source ---------------------------------- diff --git a/docs/GarbageCollection.rst b/docs/GarbageCollection.rst index f3b4bc3590..7765bd7d04 100644 --- a/docs/GarbageCollection.rst +++ b/docs/GarbageCollection.rst @@ -758,10 +758,10 @@ If ``CustomReadBarriers`` or ``CustomWriteBarriers`` are specified, then ``performCustomLowering`` **must** eliminate the corresponding barriers. ``performCustomLowering`` must comply with the same restrictions as -`FunctionPass::runOnFunction <WritingAnLLVMPass.html#runOnFunction>`__ +:ref:`FunctionPass::runOnFunction <writing-an-llvm-pass-runOnFunction>` Likewise, ``initializeCustomLowering`` has the same semantics as -`Pass::doInitialization(Module&) -<WritingAnLLVMPass.html#doInitialization_mod>`__ +:ref:`Pass::doInitialization(Module&) +<writing-an-llvm-pass-doInitialization-mod>` The following can be used as a template: diff --git a/docs/MakefileGuide.rst b/docs/MakefileGuide.rst index 2c1d33e962..168b0b3348 100644 --- a/docs/MakefileGuide.rst +++ b/docs/MakefileGuide.rst @@ -170,9 +170,9 @@ openable with the ``dlopen`` function and searchable with the ``dlsym`` function (or your operating system's equivalents). While this isn't strictly necessary on Linux and a few other platforms, it is required on systems like HP-UX and Darwin. You should use ``LOADABLE_MODULE`` for any shared library that you -intend to be loaded into an tool via the ``-load`` option. See the -`WritingAnLLVMPass.html <WritingAnLLVMPass.html#makefile>`_ document for an -example of why you might want to do this. +intend to be loaded into an tool via the ``-load`` option. `Pass documentation +<writing-an-llvm-pass-makefile>`_ has an example of why you might want to do +this. Bitcode Modules ^^^^^^^^^^^^^^^ diff --git a/docs/WritingAnLLVMBackend.rst b/docs/WritingAnLLVMBackend.rst index 7803163ae6..7e243fa3ec 100644 --- a/docs/WritingAnLLVMBackend.rst +++ b/docs/WritingAnLLVMBackend.rst @@ -54,8 +54,8 @@ These essential documents must be read before reading this document: file (``.td`` suffix) and generates C++ code that can be used for code generation. -* `Writing an LLVM Pass <WritingAnLLVMPass.html>`_ --- The assembly printer is - a ``FunctionPass``, as are several SelectionDAG processing steps. +* :doc:`WritingAnLLVMPass` --- The assembly printer is a ``FunctionPass``, as + are several ``SelectionDAG`` processing steps. To follow the SPARC examples in this document, have a copy of `The SPARC Architecture Manual, Version 8 <http://www.sparc.org/standards/V8.pdf>`_ for diff --git a/docs/WritingAnLLVMPass.html b/docs/WritingAnLLVMPass.html deleted file mode 100644 index af1ffa4fb7..0000000000 --- a/docs/WritingAnLLVMPass.html +++ /dev/null @@ -1,1954 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> - <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> - <title>Writing an LLVM Pass</title> - <link rel="stylesheet" href="_static/llvm.css" type="text/css"> -</head> -<body> - -<h1> - Writing an LLVM Pass -</h1> - -<ol> - <li><a href="#introduction">Introduction - What is a pass?</a></li> - <li><a href="#quickstart">Quick Start - Writing hello world</a> - <ul> - <li><a href="#makefile">Setting up the build environment</a></li> - <li><a href="#basiccode">Basic code required</a></li> - <li><a href="#running">Running a pass with <tt>opt</tt></a></li> - </ul></li> - <li><a href="#passtype">Pass classes and requirements</a> - <ul> - <li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li> - <li><a href="#ModulePass">The <tt>ModulePass</tt> class</a> - <ul> - <li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li> - </ul></li> - <li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a> - <ul> - <li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph - &)</tt> method</a></li> - <li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li> - <li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph - &)</tt> method</a></li> - </ul></li> - <li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a> - <ul> - <li><a href="#doInitialization_mod">The <tt>doInitialization(Module - &)</tt> method</a></li> - <li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li> - <li><a href="#doFinalization_mod">The <tt>doFinalization(Module - &)</tt> method</a></li> - </ul></li> - <li><a href="#LoopPass">The <tt>LoopPass</tt> class</a> - <ul> - <li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *, - LPPassManager &)</tt> method</a></li> - <li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li> - <li><a href="#doFinalization_loop">The <tt>doFinalization() - </tt> method</a></li> - </ul></li> - <li><a href="#RegionPass">The <tt>RegionPass</tt> class</a> - <ul> - <li><a href="#doInitialization_region">The <tt>doInitialization(Region *, - RGPassManager &)</tt> method</a></li> - <li><a href="#runOnRegion">The <tt>runOnRegion</tt> method</a></li> - <li><a href="#doFinalization_region">The <tt>doFinalization() - </tt> method</a></li> - </ul></li> - <li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a> - <ul> - <li><a href="#doInitialization_fn">The <tt>doInitialization(Function - &)</tt> method</a></li> - <li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt> - method</a></li> - <li><a href="#doFinalization_fn">The <tt>doFinalization(Function - &)</tt> method</a></li> - </ul></li> - <li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt> - class</a> - <ul> - <li><a href="#runOnMachineFunction">The - <tt>runOnMachineFunction(MachineFunction &)</tt> method</a></li> - </ul></li> - </ul> - <li><a href="#registration">Pass Registration</a> - <ul> - <li><a href="#print">The <tt>print</tt> method</a></li> - </ul></li> - <li><a href="#interaction">Specifying interactions between passes</a> - <ul> - <li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt> - method</a></li> - <li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li> - <li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li> - <li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li> - <li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and -<tt>getAnalysisIfAvailable<></tt> methods</a></li> - </ul></li> - <li><a href="#analysisgroup">Implementing Analysis Groups</a> - <ul> - <li><a href="#agconcepts">Analysis Group Concepts</a></li> - <li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li> - </ul></li> - <li><a href="#passStatistics">Pass Statistics</a> - <li><a href="#passmanager">What PassManager does</a> - <ul> - <li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li> - </ul></li> - <li><a href="#registering">Registering dynamically loaded passes</a> - <ul> - <li><a href="#registering_existing">Using existing registries</a></li> - <li><a href="#registering_new">Creating new registries</a></li> - </ul></li> - <li><a href="#debughints">Using GDB with dynamically loaded passes</a> - <ul> - <li><a href="#breakpoint">Setting a breakpoint in your pass</a></li> - <li><a href="#debugmisc">Miscellaneous Problems</a></li> - </ul></li> - <li><a href="#future">Future extensions planned</a> - <ul> - <li><a href="#SMP">Multithreaded LLVM</a></li> - </ul></li> -</ol> - -<div class="doc_author"> - <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and - <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p> -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="introduction">Introduction - What is a pass?</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM -passes are where most of the interesting parts of the compiler exist. Passes -perform the transformations and optimizations that make up the compiler, they -build the analysis results that are used by these transformations, and they are, -above all, a structuring technique for compiler code.</p> - -<p>All LLVM passes are subclasses of the <tt><a -href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt> -class, which implement functionality by overriding virtual methods inherited -from <tt>Pass</tt>. Depending on how your pass works, you should inherit from -the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a -href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a -href="#FunctionPass">FunctionPass</a></tt>, or <tt><a -href="#LoopPass">LoopPass</a></tt>, or <tt><a -href="#RegionPass">RegionPass</a></tt>, or <tt><a -href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system -more information about what your pass does, and how it can be combined with -other passes. One of the main features of the LLVM Pass Framework is that it -schedules passes to run in an efficient way based on the constraints that your -pass meets (which are indicated by which class they derive from).</p> - -<p>We start by showing you how to construct a pass, everything from setting up -the code, to compiling, loading, and executing it. After the basics are down, -more advanced features are discussed.</p> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="quickstart">Quick Start - Writing hello world</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>Here we describe how to write the "hello world" of passes. The "Hello" pass -is designed to simply print out the name of non-external functions that exist in -the program being compiled. It does not modify the program at all, it just -inspects it. The source code and files for this pass are available in the LLVM -source tree in the <tt>lib/Transforms/Hello</tt> directory.</p> - -<!-- ======================================================================= --> -<h3> - <a name="makefile">Setting up the build environment</a> -</h3> - -<div> - - <p>First, configure and build LLVM. This needs to be done directly inside the - LLVM source tree rather than in a separate objects directory. - Next, you need to create a new directory somewhere in the LLVM source - base. For this example, we'll assume that you made - <tt>lib/Transforms/Hello</tt>. Finally, you must set up a build script - (Makefile) that will compile the source code for the new pass. To do this, - copy the following into <tt>Makefile</tt>:</p> - <hr> - -<div class="doc_code"><pre> -# Makefile for hello pass - -# Path to top level of LLVM hierarchy -LEVEL = ../../.. - -# Name of the library to build -LIBRARYNAME = Hello - -# Make the shared library become a loadable module so the tools can -# dlopen/dlsym on the resulting library. -LOADABLE_MODULE = 1 - -# Include the makefile implementation stuff -include $(LEVEL)/Makefile.common -</pre></div> - -<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current -directory are to be compiled and linked together into a shared object -<tt>$(LEVEL)/Debug+Asserts/lib/Hello.so</tt> that can be dynamically loaded by -the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options. -If your operating system uses a suffix other than .so (such as windows or -Mac OS/X), the appropriate extension will be used.</p> - -<p>If you are used CMake to build LLVM, see -<a href="CMake.html#passdev">Developing an LLVM pass with CMake</a>.</p> - -<p>Now that we have the build scripts set up, we just need to write the code for -the pass itself.</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="basiccode">Basic code required</a> -</h3> - -<div> - -<p>Now that we have a way to compile our new pass, we just have to write it. -Start out with:</p> - -<div class="doc_code"> -<pre> -<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>" -<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>" -<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>" -</pre> -</div> - -<p>Which are needed because we are writing a <tt><a -href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, -we are operating on <tt><a -href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s, -and we will be doing some printing.</p> - -<p>Next we have:</p> - -<div class="doc_code"> -<pre> -<b>using namespace llvm;</b> -</pre> -</div> - -<p>... which is required because the functions from the include files -live in the llvm namespace.</p> - -<p>Next we have:</p> - -<div class="doc_code"> -<pre> -<b>namespace</b> { -</pre> -</div> - -<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++ -what the "<tt>static</tt>" keyword is to C (at global scope). It makes the -things declared inside of the anonymous namespace visible only to the current -file. If you're not familiar with them, consult a decent C++ book for more -information.</p> - -<p>Next, we declare our pass itself:</p> - -<div class="doc_code"> -<pre> - <b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> { -</pre> -</div> - -<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a -href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>. -The different builtin pass subclasses are described in detail <a -href="#passtype">later</a>, but for now, know that <a -href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate on a function at a -time.</p> - -<div class="doc_code"> -<pre> - static char ID; - Hello() : FunctionPass(ID) {} -</pre> -</div> - -<p>This declares pass identifier used by LLVM to identify pass. This allows LLVM -to avoid using expensive C++ runtime information.</p> - -<div class="doc_code"> -<pre> - <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) { - errs() << "<i>Hello: </i>"; - errs().write_escaped(F.getName()) << "\n"; - <b>return false</b>; - } - }; <i>// end of struct Hello</i> -} <i>// end of anonymous namespace</i> -</pre> -</div> - -<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method, -which overloads an abstract virtual method inherited from <a -href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed -to do our thing, so we just print out our message with the name of each -function.</p> - -<div class="doc_code"> -<pre> -char Hello::ID = 0; -</pre> -</div> - -<p>We initialize pass ID here. LLVM uses ID's address to identify a pass, so -initialization value is not important.</p> - -<div class="doc_code"> -<pre> -static RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>", - false /* Only looks at CFG */, - false /* Analysis Pass */); -</pre> -</div> - -<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>, -giving it a command line argument "<tt>hello</tt>", and a name "<tt>Hello World -Pass</tt>". The last two arguments describe its behavior: if a pass walks CFG -without modifying it then the third argument is set to <tt>true</tt>; if a pass -is an analysis pass, for example dominator tree pass, then <tt>true</tt> is -supplied as the fourth argument.</p> - -<p>As a whole, the <tt>.cpp</tt> file looks like:</p> - -<div class="doc_code"> -<pre> -<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>" -<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>" -<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>" - -<b>using namespace llvm;</b> - -<b>namespace</b> { - <b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> { - - static char ID; - Hello() : FunctionPass(ID) {} - - <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) { - errs() << "<i>Hello: </i>"; - errs().write_escaped(F.getName()) << '\n'; - <b>return false</b>; - } - - }; -} - -char Hello::ID = 0; -static RegisterPass<Hello> X("hello", "Hello World Pass", false, false); -</pre> -</div> - -<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>" -command in the local directory and you should get a new file -"<tt>Debug+Asserts/lib/Hello.so</tt>" under the top level directory of the LLVM -source tree (not in the local directory). Note that everything in this file is -contained in an anonymous namespace — this reflects the fact that passes -are self contained units that do not need external interfaces (although they can -have them) to be useful.</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="running">Running a pass with <tt>opt</tt></a> -</h3> - -<div> - -<p>Now that you have a brand new shiny shared object file, we can use the -<tt>opt</tt> command to run an LLVM program through your pass. Because you -registered your pass with <tt>RegisterPass</tt>, you will be able to -use the <tt>opt</tt> tool to access it, once loaded.</p> - -<p>To test it, follow the example at the end of the <a -href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to -LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program -through our transformation like this (or course, any bitcode file will -work):</p> - -<div class="doc_code"><pre> -$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null -Hello: __main -Hello: puts -Hello: main -</pre></div> - -<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your -pass as a shared object, which makes '<tt>-hello</tt>' a valid command line -argument (which is one reason you need to <a href="#registration">register your -pass</a>). Because the hello pass does not modify the program in any -interesting way, we just throw away the result of <tt>opt</tt> (sending it to -<tt>/dev/null</tt>).</p> - -<p>To see what happened to the other string you registered, try running -<tt>opt</tt> with the <tt>-help</tt> option:</p> - -<div class="doc_code"><pre> -$ opt -load ../../../Debug+Asserts/lib/Hello.so -help -OVERVIEW: llvm .bc -> .bc modular optimizer - -USAGE: opt [options] <input bitcode> - -OPTIONS: - Optimizations available: -... - -globalopt - Global Variable Optimizer - -globalsmodref-aa - Simple mod/ref analysis for globals - -gvn - Global Value Numbering - <b>-hello - Hello World Pass</b> - -indvars - Induction Variable Simplification - -inline - Function Integration/Inlining - -insert-edge-profiling - Insert instrumentation for edge profiling -... -</pre></div> - -<p>The pass name gets added as the information string for your pass, giving some -documentation to users of <tt>opt</tt>. Now that you have a working pass, you -would go ahead and make it do the cool transformations you want. Once you get -it all working and tested, it may become useful to find out how fast your pass -is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command -line option (<tt>--time-passes</tt>) that allows you to get information about -the execution time of your pass along with the other passes you queue up. For -example:</p> - -<div class="doc_code"><pre> -$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null -Hello: __main -Hello: puts -Hello: main -=============================================================================== - ... Pass execution timing report ... -=============================================================================== - Total Execution Time: 0.02 seconds (0.0479059 wall clock) - - ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name --- - 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer - 0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction - 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier - <b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b> - 0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL -</pre></div> - -<p>As you can see, our implementation above is pretty fast :). The additional -passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify -that the LLVM emitted by your pass is still valid and well formed LLVM, which -hasn't been broken somehow.</p> - -<p>Now that you have seen the basics of the mechanics behind passes, we can talk -about some more details of how they work and how to use them.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="passtype">Pass classes and requirements</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>One of the first things that you should do when designing a new pass is to -decide what class you should subclass for your pass. The <a -href="#basiccode">Hello World</a> example uses the <tt><a -href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we -did not discuss why or when this should occur. Here we talk about the classes -available, from the most general to the most specific.</p> - -<p>When choosing a superclass for your Pass, you should choose the <b>most -specific</b> class possible, while still being able to meet the requirements -listed. This gives the LLVM Pass Infrastructure information necessary to -optimize how passes are run, so that the resultant compiler isn't unnecessarily -slow.</p> - -<!-- ======================================================================= --> -<h3> - <a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a> -</h3> - -<div> - -<p>The most plain and boring type of pass is the "<tt><a -href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>" -class. This pass type is used for passes that do not have to be run, do not -change state, and never need to be updated. This is not a normal type of -transformation or analysis, but can provide information about the current -compiler configuration.</p> - -<p>Although this pass class is very infrequently used, it is important for -providing information about the current target machine being compiled for, and -other static information that can affect the various transformations.</p> - -<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never -invalidated, and are never "run".</p> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="ModulePass">The <tt>ModulePass</tt> class</a> -</h3> - -<div> - -<p>The "<tt><a -href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>" -class is the most general of all superclasses that you can use. Deriving from -<tt>ModulePass</tt> indicates that your pass uses the entire program as a unit, -referring to function bodies in no predictable order, or adding and removing -functions. Because nothing is known about the behavior of <tt>ModulePass</tt> -subclasses, no optimization can be done for their execution.</p> - -<p>A module pass can use function level passes (e.g. dominators) using -the getAnalysis interface -<tt>getAnalysis<DominatorTree>(llvm::Function *)</tt> to provide the -function to retrieve analysis result for, if the function pass does not require -any module or immutable passes. Note that this can only be done for functions for which the -analysis ran, e.g. in the case of dominators you should only ask for the -DominatorTree for function definitions, not declarations.</p> - -<p>To write a correct <tt>ModulePass</tt> subclass, derive from -<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the -following signature:</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="runOnModule">The <tt>runOnModule</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> runOnModule(Module &M) = 0; -</pre></div> - -<p>The <tt>runOnModule</tt> method performs the interesting work of the pass. -It should return true if the module was modified by the transformation and -false otherwise.</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a> -</h3> - -<div> - -<p>The "<tt><a -href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>" -is used by passes that need to traverse the program bottom-up on the call graph -(callees before callers). Deriving from CallGraphSCCPass provides some -mechanics for building and traversing the CallGraph, but also allows the system -to optimize execution of CallGraphSCCPass's. If your pass meets the -requirements outlined below, and doesn't meet the requirements of a <tt><a -href="#FunctionPass">FunctionPass</a></tt> or <tt><a -href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from -<tt>CallGraphSCCPass</tt>.</p> - -<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p> - -<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p> - -<ol> - -<li>... <em>not allowed</em> to inspect or modify any <tt>Function</tt>s other -than those in the current SCC and the direct callers and direct callees of the -SCC.</li> - -<li>... <em>required</em> to preserve the current CallGraph object, updating it -to reflect any changes made to the program.</li> - -<li>... <em>not allowed</em> to add or remove SCC's from the current Module, -though they may change the contents of an SCC.</li> - -<li>... <em>allowed</em> to add or remove global variables from the current -Module.</li> - -<li>... <em>allowed</em> to maintain state across invocations of - <a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li> -</ol> - -<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases -because it has to handle SCCs with more than one node in it. All of the virtual -methods described below should return true if they modified the program, or -false if they didn't.</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doInitialization_scc"> - The <tt>doInitialization(CallGraph &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doInitialization(CallGraph &CG); -</pre></div> - -<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that -<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove -functions, get pointers to functions, etc. The <tt>doInitialization</tt> method -is designed to do simple initialization type of stuff that does not depend on -the SCCs being processed. The <tt>doInitialization</tt> method call is not -scheduled to overlap with any other pass executions (thus it should be very -fast).</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="runOnSCC">The <tt>runOnSCC</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> runOnSCC(CallGraphSCC &SCC) = 0; -</pre></div> - -<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and -should return true if the module was modified by the transformation, false -otherwise.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doFinalization_scc"> - The <tt>doFinalization(CallGraph &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doFinalization(CallGraph &CG); -</pre></div> - -<p>The <tt>doFinalization</tt> method is an infrequently used method that is -called when the pass framework has finished calling <a -href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the -program being compiled.</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="FunctionPass">The <tt>FunctionPass</tt> class</a> -</h3> - -<div> - -<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a -href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt> -subclasses do have a predictable, local behavior that can be expected by the -system. All <tt>FunctionPass</tt> execute on each function in the program -independent of all of the other functions in the program. -<tt>FunctionPass</tt>'s do not require that they are executed in a particular -order, and <tt>FunctionPass</tt>'s do not modify external functions.</p> - -<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p> - -<ol> -<li>Modify a Function other than the one currently being processed.</li> -<li>Add or remove Function's from the current Module.</li> -<li>Add or remove global variables from the current Module.</li> -<li>Maintain state across invocations of - <a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li> -</ol> - -<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a -href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s -may overload three virtual methods to do their work. All of these methods -should return true if they modified the program, or false if they didn't.</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doInitialization_mod"> - The <tt>doInitialization(Module &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doInitialization(Module &M); -</pre></div> - -<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that -<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove -functions, get pointers to functions, etc. The <tt>doInitialization</tt> method -is designed to do simple initialization type of stuff that does not depend on -the functions being processed. The <tt>doInitialization</tt> method call is not -scheduled to overlap with any other pass executions (thus it should be very -fast).</p> - -<p>A good example of how this method should be used is the <a -href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a> -pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into -platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It -uses the <tt>doInitialization</tt> method to get a reference to the malloc and -free functions that it needs, adding prototypes to the module if necessary.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="runOnFunction">The <tt>runOnFunction</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> runOnFunction(Function &F) = 0; -</pre></div><p> - -<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do -the transformation or analysis work of your pass. As usual, a true value should -be returned if the function is modified.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doFinalization_mod"> - The <tt>doFinalization(Module &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doFinalization(Module &M); -</pre></div> - -<p>The <tt>doFinalization</tt> method is an infrequently used method that is -called when the pass framework has finished calling <a -href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the -program being compiled.</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="LoopPass">The <tt>LoopPass</tt> class </a> -</h3> - -<div> - -<p> All <tt>LoopPass</tt> execute on each loop in the function independent of -all of the other loops in the function. <tt>LoopPass</tt> processes loops in -loop nest order such that outer most loop is processed last. </p> - -<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using -<tt>LPPassManager</tt> interface. Implementing a loop pass is usually -straightforward. <tt>LoopPass</tt>'s may overload three virtual methods to -do their work. All these methods should return true if they modified the -program, or false if they didn't. </p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doInitialization_loop"> - The <tt>doInitialization(Loop *,LPPassManager &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM); -</pre></div> - -<p>The <tt>doInitialization</tt> method is designed to do simple initialization -type of stuff that does not depend on the functions being processed. The -<tt>doInitialization</tt> method call is not scheduled to overlap with any -other pass executions (thus it should be very fast). LPPassManager -interface should be used to access Function or Module level analysis -information.</p> - -</div> - - -<!-- _______________________________________________________________________ --> -<h4> - <a name="runOnLoop">The <tt>runOnLoop</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0; -</pre></div><p> - -<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do -the transformation or analysis work of your pass. As usual, a true value should -be returned if the function is modified. <tt>LPPassManager</tt> interface -should be used to update loop nest.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doFinalization(); -</pre></div> - -<p>The <tt>doFinalization</tt> method is an infrequently used method that is -called when the pass framework has finished calling <a -href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the -program being compiled. </p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="RegionPass">The <tt>RegionPass</tt> class </a> -</h3> - -<div> - -<p> <tt>RegionPass</tt> is similar to <a href="#LoopPass"><tt>LoopPass</tt></a>, -but executes on each single entry single exit region in the function. -<tt>RegionPass</tt> processes regions in nested order such that the outer most -region is processed last. </p> - -<p> <tt>RegionPass</tt> subclasses are allowed to update the region tree by using -the <tt>RGPassManager</tt> interface. You may overload three virtual methods of -<tt>RegionPass</tt> to implement your own region pass. All these -methods should return true if they modified the program, or false if they didn not. -</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doInitialization_region"> - The <tt>doInitialization(Region *, RGPassManager &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doInitialization(Region *, RGPassManager &RGM); -</pre></div> - -<p>The <tt>doInitialization</tt> method is designed to do simple initialization -type of stuff that does not depend on the functions being processed. The -<tt>doInitialization</tt> method call is not scheduled to overlap with any -other pass executions (thus it should be very fast). RPPassManager -interface should be used to access Function or Module level analysis -information.</p> - -</div> - - -<!-- _______________________________________________________________________ --> -<h4> - <a name="runOnRegion">The <tt>runOnRegion</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> runOnRegion(Region *, RGPassManager &RGM) = 0; -</pre></div><p> - -<p>The <tt>runOnRegion</tt> method must be implemented by your subclass to do -the transformation or analysis work of your pass. As usual, a true value should -be returned if the region is modified. <tt>RGPassManager</tt> interface -should be used to update region tree.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doFinalization_region">The <tt>doFinalization()</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doFinalization(); -</pre></div> - -<p>The <tt>doFinalization</tt> method is an infrequently used method that is -called when the pass framework has finished calling <a -href="#runOnRegion"><tt>runOnRegion</tt></a> for every region in the -program being compiled. </p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a> -</h3> - -<div> - -<p><tt>BasicBlockPass</tt>'s are just like <a -href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit -their scope of inspection and modification to a single basic block at a time. -As such, they are <b>not</b> allowed to do any of the following:</p> - -<ol> -<li>Modify or inspect any basic blocks outside of the current one</li> -<li>Maintain state across invocations of - <a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li> -<li>Modify the control flow graph (by altering terminator instructions)</li> -<li>Any of the things forbidden for - <a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li> -</ol> - -<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole" -optimizations. They may override the same <a -href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a -href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a -href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doInitialization_fn"> - The <tt>doInitialization(Function &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doInitialization(Function &F); -</pre></div> - -<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that -<tt>BasicBlockPass</tt>'s are not allowed to do, but that -<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed -to do simple initialization that does not depend on the -BasicBlocks being processed. The <tt>doInitialization</tt> method call is not -scheduled to overlap with any other pass executions (thus it should be very -fast).</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0; -</pre></div> - -<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This -function is not allowed to inspect or modify basic blocks other than the -parameter, and are not allowed to modify the CFG. A true value must be returned -if the basic block is modified.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="doFinalization_fn"> - The <tt>doFinalization(Function &)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> doFinalization(Function &F); -</pre></div> - -<p>The <tt>doFinalization</tt> method is an infrequently used method that is -called when the pass framework has finished calling <a -href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the -program being compiled. This can be used to perform per-function -finalization.</p> - -</div> - -</div> - -<!-- ======================================================================= --> -<h3> - <a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a> -</h3> - -<div> - -<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that -executes on the machine-dependent representation of each LLVM function in the -program.</p> - -<p>Code generator passes are registered and initialized specially by -<tt>TargetMachine::addPassesToEmitFile</tt> and similar routines, so they -cannot generally be run from the <tt>opt</tt> or <tt>bugpoint</tt> -commands.</p> - -<p>A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all -the restrictions that apply to a <tt>FunctionPass</tt> also apply to it. -<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular, -<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p> - -<ol> -<li>Modify or create any LLVM IR Instructions, BasicBlocks, Arguments, - Functions, GlobalVariables, GlobalAliases, or Modules.</li> -<li>Modify a MachineFunction other than the one currently being processed.</li> -<li>Maintain state across invocations of <a -href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global -data)</li> -</ol> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="runOnMachineFunction"> - The <tt>runOnMachineFunction(MachineFunction &MF)</tt> method - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0; -</pre></div> - -<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a -<tt>MachineFunctionPass</tt>; that is, you should override this method to do the -work of your <tt>MachineFunctionPass</tt>.</p> - -<p>The <tt>runOnMachineFunction</tt> method is called on every -<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the -<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent -representation of the function. If you want to get at the LLVM <tt>Function</tt> -for the <tt>MachineFunction</tt> you're working on, use -<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but -remember, you may not modify the LLVM <tt>Function</tt> or its contents from a -<tt>MachineFunctionPass</tt>.</p> - -</div> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="registration">Pass registration</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how -pass registration works, and discussed some of the reasons that it is used and -what it does. Here we discuss how and why passes are registered.</p> - -<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b> -template. The template parameter is the name of the pass that is to be used on -the command line to specify that the pass should be added to a program (for -example, with <tt>opt</tt> or <tt>bugpoint</tt>). The first argument is the -name of the pass, which is to be used for the <tt>-help</tt> output of -programs, as -well as for debug output generated by the <tt>--debug-pass</tt> option.</p> - -<p>If you want your pass to be easily dumpable, you should -implement the virtual <tt>print</tt> method:</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="print">The <tt>print</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>; -</pre></div> - -<p>The <tt>print</tt> method must be implemented by "analyses" in order to print -a human readable version of the analysis results. This is useful for debugging -an analysis itself, as well as for other people to figure out how an analysis -works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p> - -<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on, -and the <tt>Module</tt> parameter gives a pointer to the top level module of the -program that has been analyzed. Note however that this pointer may be null in -certain circumstances (such as calling the <tt>Pass::dump()</tt> from a -debugger), so it should only be used to enhance debug output, it should not be -depended on.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="interaction">Specifying interactions between passes</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>One of the main responsibilities of the <tt>PassManager</tt> is to make sure -that passes interact with each other correctly. Because <tt>PassManager</tt> -tries to <a href="#passmanager">optimize the execution of passes</a> it must -know how the passes interact with each other and what dependencies exist between -the various passes. To track this, each pass can declare the set of passes that -are required to be executed before the current pass, and the passes which are -invalidated by the current pass.</p> - -<p>Typically this functionality is used to require that analysis results are -computed before your pass is run. Running arbitrary transformation passes can -invalidate the computed analysis results, which is what the invalidation set -specifies. If a pass does not implement the <tt><a -href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not -having any prerequisite passes, and invalidating <b>all</b> other passes.</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> -<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>; -</pre></div> - -<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and -invalidated sets may be specified for your transformation. The implementation -should fill in the <tt><a -href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt> -object with information about which passes are required and not invalidated. To -do this, a pass may call any of the following methods on the AnalysisUsage -object:</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="AU::addRequired"> - The <tt>AnalysisUsage::addRequired<></tt> - and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods - </a> -</h4> - -<div> -<p> -If your pass requires a previous pass to be executed (an analysis for example), -it can use one of these methods to arrange for it to be run before your pass. -LLVM has many different types of analyses and passes that can be required, -spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>. -Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will -be no critical edges in the CFG when your pass has been run. -</p> - -<p> -Some analyses chain to other analyses to do their job. For example, an <a -href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a -href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In -cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be -used instead of the <tt>addRequired</tt> method. This informs the PassManager -that the transitively required pass should be alive as long as the requiring -pass is. -</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="AU::addPreserved"> - The <tt>AnalysisUsage::addPreserved<></tt> method - </a> -</h4> - -<div> -<p> -One of the jobs of the PassManager is to optimize how and when analyses are run. -In particular, it attempts to avoid recomputing data unless it needs to. For -this reason, passes are allowed to declare that they preserve (i.e., they don't -invalidate) an existing analysis if it's available. For example, a simple -constant folding pass would not modify the CFG, so it can't possibly affect the -results of dominator analysis. By default, all passes are assumed to invalidate -all others. -</p> - -<p> -The <tt>AnalysisUsage</tt> class provides several methods which are useful in -certain circumstances that are related to <tt>addPreserved</tt>. In particular, -the <tt>setPreservesAll</tt> method can be called to indicate that the pass does -not modify the LLVM program at all (which is true for analyses), and the -<tt>setPreservesCFG</tt> method can be used by transformations that change -instructions in the program but do not modify the CFG or terminator instructions -(note that this property is implicitly set for <a -href="#BasicBlockPass">BasicBlockPass</a>'s). -</p> - -<p> -<tt>addPreserved</tt> is particularly useful for transformations like -<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop -and dominator related analyses if they exist, so it can preserve them, despite -the fact that it hacks on the CFG. -</p> -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="AU::examples"> - Example implementations of <tt>getAnalysisUsage</tt> - </a> -</h4> - -<div> - -<div class="doc_code"><pre> -<i>// This example modifies the program, but does not modify the CFG</i> -<b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> { - AU.setPreservesCFG(); - AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>(); -} -</pre></div> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="getAnalysis"> - The <tt>getAnalysis<></tt> and - <tt>getAnalysisIfAvailable<></tt> methods - </a> -</h4> - -<div> - -<p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by -your class, providing you with access to the passes that you declared that you -required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> -method. It takes a single template argument that specifies which pass class you -want, and returns a reference to that pass. For example:</p> - -<div class="doc_code"><pre> -bool LICM::runOnFunction(Function &F) { - LoopInfo &LI = getAnalysis<LoopInfo>(); - ... -} -</pre></div> - -<p>This method call returns a reference to the pass desired. You may get a -runtime assertion failure if you attempt to get an analysis that you did not -declare as required in your <a -href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This -method can be called by your <tt>run*</tt> method implementation, or by any -other local method invoked by your <tt>run*</tt> method. - -A module level pass can use function level analysis info using this interface. -For example:</p> - -<div class="doc_code"><pre> -bool ModuleLevelPass::runOnModule(Module &M) { - ... - DominatorTree &DT = getAnalysis<DominatorTree>(Func); - ... -} -</pre></div> - -<p>In above example, runOnFunction for DominatorTree is called by pass manager -before returning a reference to the desired pass.</p> - -<p> -If your pass is capable of updating analyses if they exist (e.g., -<tt>BreakCriticalEdges</tt>, as described above), you can use the -<tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis -if it is active. For example:</p> - -<div class="doc_code"><pre> -... -if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) { - <i>// A DominatorSet is active. This code will update it.</i> -} -... -</pre></div> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="analysisgroup">Implementing Analysis Groups</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>Now that we understand the basics of how passes are defined, how they are -used, and how they are required from other passes, it's time to get a little bit -fancier. All of the pass relationships that we have seen so far are very -simple: one pass depends on one other specific pass to be run before it can run. -For many applications, this is great, for others, more flexibility is -required.</p> - -<p>In particular, some analyses are defined such that there is a single simple -interface to the analysis results, but multiple ways of calculating them. -Consider alias analysis for example. The most trivial alias analysis returns -"may alias" for any alias query. The most sophisticated analysis a -flow-sensitive, context-sensitive interprocedural analysis that can take a -significant amount of time to execute (and obviously, there is a lot of room -between these two extremes for other implementations). To cleanly support -situations like this, the LLVM Pass Infrastructure supports the notion of -Analysis Groups.</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="agconcepts">Analysis Group Concepts</a> -</h4> - -<div> - -<p>An Analysis Group is a single simple interface that may be implemented by -multiple different passes. Analysis Groups can be given human readable names -just like passes, but unlike passes, they need not derive from the <tt>Pass</tt> -class. An analysis group may have one or more implementations, one of which is -the "default" implementation.</p> - -<p>Analysis groups are used by client passes just like other passes are: the -<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods. -In order to resolve this requirement, the <a href="#passmanager">PassManager</a> -scans the available passes to see if any implementations of the analysis group -are available. If none is available, the default implementation is created for -the pass to use. All standard rules for <A href="#interaction">interaction -between passes</a> still apply.</p> - -<p>Although <a href="#registration">Pass Registration</a> is optional for normal -passes, all analysis group implementations must be registered, and must use the -<A href="#registerag"><tt>INITIALIZE_AG_PASS</tt></a> template to join the -implementation pool. Also, a default implementation of the interface -<b>must</b> be registered with <A -href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p> - -<p>As a concrete example of an Analysis Group in action, consider the <a -href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> -analysis group. The default implementation of the alias analysis interface (the -<tt><a -href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt> -pass) just does a few simple checks that don't require significant analysis to -compute (such as: two different globals can never alias each other, etc). -Passes that use the <tt><a -href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt> -interface (for example the <tt><a -href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do -not care which implementation of alias analysis is actually provided, they just -use the designated interface.</p> - -<p>From the user's perspective, commands work just like normal. Issuing the -command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be -instantiated and added to the pass sequence. Issuing the command '<tt>opt --somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the -<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a -hypothetical example) instead.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a> -</h4> - -<div> - -<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis -group itself, while the <tt>INITIALIZE_AG_PASS</tt> is used to add pass -implementations to the analysis group. First, -an analysis group should be registered, with a human readable name -provided for it. -Unlike registration of passes, there is no command line argument to be specified -for the Analysis Group Interface itself, because it is "abstract":</p> - -<div class="doc_code"><pre> -<b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>"); -</pre></div> - -<p>Once the analysis is registered, passes can declare that they are valid -implementations of the interface by using the following code:</p> - -<div class="doc_code"><pre> -<b>namespace</b> { - //<i> Declare that we implement the AliasAnalysis interface</i> - INITIALIZE_AG_PASS(FancyAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>somefancyaa</i>", - "<i>A more complex alias analysis implementation</i>", - false, // <i>Is CFG Only?</i> - true, // <i>Is Analysis?</i> - false); // <i>Is default Analysis Group implementation?</i> -} -</pre></div> - -<p>This just shows a class <tt>FancyAA</tt> that -uses the <tt>INITIALIZE_AG_PASS</tt> macro both to register and -to "join" the <tt><a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt> -analysis group. Every implementation of an analysis group should join using -this macro.</p> - -<div class="doc_code"><pre> -<b>namespace</b> { - //<i> Declare that we implement the AliasAnalysis interface</i> - INITIALIZE_AG_PASS(BasicAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>basicaa</i>", - "<i>Basic Alias Analysis (default AA impl)</i>", - false, // <i>Is CFG Only?</i> - true, // <i>Is Analysis?</i> - true); // <i>Is default Analysis Group implementation?</i> -} -</pre></div> - -<p>Here we show how the default implementation is specified (using the final -argument to the <tt>INITIALIZE_AG_PASS</tt> template). There must be exactly -one default implementation available at all times for an Analysis Group to be -used. Only default implementation can derive from <tt>ImmutablePass</tt>. -Here we declare that the - <tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt> -pass is the default implementation for the interface.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="passStatistics">Pass Statistics</a> -</h2> -<!-- *********************************************************************** --> - -<div> -<p>The <a -href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a> -class is designed to be an easy way to expose various success -metrics from passes. These statistics are printed at the end of a -run, when the -stats command line option is enabled on the command -line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details. - -</div> - - -<!-- *********************************************************************** --> -<h2> - <a name="passmanager">What PassManager does</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>The <a -href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a> -<a -href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a> -takes a list of passes, ensures their <a href="#interaction">prerequisites</a> -are set up correctly, and then schedules passes to run efficiently. All of the -LLVM tools that run passes use the <tt>PassManager</tt> for execution of these -passes.</p> - -<p>The <tt>PassManager</tt> does two main things to try to reduce the execution -time of a series of passes:</p> - -<ol> -<li><b>Share analysis results</b> - The PassManager attempts to avoid -recomputing analysis results as much as possible. This means keeping track of -which analyses are available already, which analyses get invalidated, and which -analyses are needed to be run for a pass. An important part of work is that the -<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing -it to <a href="#releaseMemory">free memory</a> allocated to holding analysis -results as soon as they are no longer needed.</li> - -<li><b>Pipeline the execution of passes on the program</b> - The -<tt>PassManager</tt> attempts to get better cache and memory usage behavior out -of a series of passes by pipelining the passes together. This means that, given -a series of consecutive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it -will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on -the first function, then all of the <a -href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function, -etc... until the entire program has been run through the passes. - -<p>This improves the cache behavior of the compiler, because it is only touching -the LLVM program representation for a single function at a time, instead of -traversing the entire program. It reduces the memory consumption of compiler, -because, for example, only one <a -href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a> -needs to be calculated at a time. This also makes it possible to implement -some <a -href="#SMP">interesting enhancements</a> in the future.</p></li> - -</ol> - -<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how -much information it has about the behaviors of the passes it is scheduling. For -example, the "preserved" set is intentionally conservative in the face of an -unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method. -Not implementing when it should be implemented will have the effect of not -allowing any analysis results to live across the execution of your pass.</p> - -<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line -options that is useful for debugging pass execution, seeing how things work, and -diagnosing when you should be preserving more analyses than you currently are -(To get information about all of the variants of the <tt>--debug-pass</tt> -option, just type '<tt>opt -help-hidden</tt>').</p> - -<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see -how our <a href="#basiccode">Hello World</a> pass interacts with other passes. -Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p> - -<div class="doc_code"><pre> -$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null -Module Pass Manager - Function Pass Manager - Dominator Set Construction - Immediate Dominators Construction - Global Common Subexpression Elimination --- Immediate Dominators Construction --- Global Common Subexpression Elimination - Natural Loop Construction - Loop Invariant Code Motion --- Natural Loop Construction --- Loop Invariant Code Motion - Module Verifier --- Dominator Set Construction --- Module Verifier - Bitcode Writer ---Bitcode Writer -</pre></div> - -<p>This output shows us when passes are constructed and when the analysis -results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that -GCSE uses dominator and immediate dominator information to do its job. The LICM -pass uses natural loop information, which uses dominator sets, but not immediate -dominators. Because immediate dominators are no longer useful after the GCSE -pass, it is immediately destroyed. The dominator sets are then reused to -compute natural loop information, which is then used by the LICM pass.</p> - -<p>After the LICM pass, the module verifier runs (which is automatically added -by the '<tt>opt</tt>' tool), which uses the dominator set to check that the -resultant LLVM code is well formed. After it finishes, the dominator set -information is destroyed, after being computed once, and shared by three -passes.</p> - -<p>Lets see how this changes when we run the <a href="#basiccode">Hello -World</a> pass in between the two passes:</p> - -<div class="doc_code"><pre> -$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null -Module Pass Manager - Function Pass Manager - Dominator Set Construction - Immediate Dominators Construction - Global Common Subexpression Elimination -<b>-- Dominator Set Construction</b> --- Immediate Dominators Construction --- Global Common Subexpression Elimination -<b> Hello World Pass --- Hello World Pass - Dominator Set Construction</b> - Natural Loop Construction - Loop Invariant Code Motion --- Natural Loop Construction --- Loop Invariant Code Motion - Module Verifier --- Dominator Set Construction --- Module Verifier - Bitcode Writer ---Bitcode Writer -Hello: __main -Hello: puts -Hello: main -</pre></div> - -<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the -Dominator Set pass, even though it doesn't modify the code at all! To fix this, -we need to add the following <a -href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p> - -<div class="doc_code"><pre> -<i>// We don't modify the program, so we preserve all analyses</i> -<b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> { - AU.setPreservesAll(); -} -</pre></div> - -<p>Now when we run our pass, we get this output:</p> - -<div class="doc_code"><pre> -$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null -Pass Arguments: -gcse -hello -licm -Module Pass Manager - Function Pass Manager - Dominator Set Construction - Immediate Dominators Construction - Global Common Subexpression Elimination --- Immediate Dominators Construction --- Global Common Subexpression Elimination - Hello World Pass --- Hello World Pass - Natural Loop Construction - Loop Invariant Code Motion --- Loop Invariant Code Motion --- Natural Loop Construction - Module Verifier --- Dominator Set Construction --- Module Verifier - Bitcode Writer ---Bitcode Writer -Hello: __main -Hello: puts -Hello: main -</pre></div> - -<p>Which shows that we don't accidentally invalidate dominator information -anymore, and therefore do not have to compute it twice.</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="releaseMemory">The <tt>releaseMemory</tt> method</a> -</h4> - -<div> - -<div class="doc_code"><pre> - <b>virtual void</b> releaseMemory(); -</pre></div> - -<p>The <tt>PassManager</tt> automatically determines when to compute analysis -results, and how long to keep them around for. Because the lifetime of the pass -object itself is effectively the entire duration of the compilation process, we -need some way to free analysis results when they are no longer useful. The -<tt>releaseMemory</tt> virtual method is the way to do this.</p> - -<p>If you are writing an analysis or any other pass that retains a significant -amount of state (for use by another pass which "requires" your pass and uses the -<a href="#getAnalysis">getAnalysis</a> method) you should implement -<tt>releaseMemory</tt> to, well, release the memory allocated to maintain this -internal state. This method is called after the <tt>run*</tt> method for the -class, before the next call of <tt>run*</tt> in your pass.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="registering">Registering dynamically loaded passes</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p><i>Size matters</i> when constructing production quality tools using llvm, -both for the purposes of distribution, and for regulating the resident code size -when running on the target system. Therefore, it becomes desirable to -selectively use some passes, while omitting others and maintain the flexibility -to change configurations later on. You want to be able to do all this, and, -provide feedback to the user. This is where pass registration comes into -play.</p> - -<p>The fundamental mechanisms for pass registration are the -<tt>MachinePassRegistry</tt> class and subclasses of -<tt>MachinePassRegistryNode</tt>.</p> - -<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of -<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and -communicates additions and deletions to the command line interface.</p> - -<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain -information provided about a particular pass. This information includes the -command line name, the command help string and the address of the function used -to create an instance of the pass. A global static constructor of one of these -instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>, -the static destructor <i>unregisters</i>. Thus a pass that is statically linked -in the tool will be registered at start up. A dynamically loaded pass will -register on load and unregister at unload.</p> - -<!-- _______________________________________________________________________ --> -<h3> - <a name="registering_existing">Using existing registries</a> -</h3> - -<div> - -<p>There are predefined registries to track instruction scheduling -(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>) -machine passes. Here we will describe how to <i>register</i> a register -allocator machine pass.</p> - -<p>Implement your register allocator machine pass. In your register allocator -<tt>.cpp</tt> file add the following include;</p> - -<div class="doc_code"><pre> -#include "llvm/CodeGen/RegAllocRegistry.h" -</pre></div> - -<p>Also in your register allocator .cpp file, define a creator function in the -form; </p> - -<div class="doc_code"><pre> -FunctionPass *createMyRegisterAllocator() { - return new MyRegisterAllocator(); -} -</pre></div> - -<p>Note that the signature of this function should match the type of -<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the -"installing" declaration, in the form;</p> - -<div class="doc_code"><pre> -static RegisterRegAlloc myRegAlloc("myregalloc", - "my register allocator help string", - createMyRegisterAllocator); -</pre></div> - -<p>Note the two spaces prior to the help string produces a tidy result on the --help query.</p> - -<div class="doc_code"><pre> -$ llc -help - ... - -regalloc - Register allocator to use (default=linearscan) - =linearscan - linear scan register allocator - =local - local register allocator - =simple - simple register allocator - =myregalloc - my register allocator help string - ... -</pre></div> - -<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as -an option. Registering instruction schedulers is similar except use the -<tt>RegisterScheduler</tt> class. Note that the -<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from -<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p> - -<p>To force the load/linking of your register allocator into the llc/lli tools, -add your creator function's global declaration to "Passes.h" and add a "pseudo" -call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p> - -</div> - - -<!-- _______________________________________________________________________ --> -<h3> - <a name="registering_new">Creating new registries</a> -</h3> - -<div> - -<p>The easiest way to get started is to clone one of the existing registries; we -recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify -are the class name and the <tt>FunctionPassCtor</tt> type.</p> - -<p>Then you need to declare the registry. Example: if your pass registry is -<tt>RegisterMyPasses</tt> then define;</p> - -<div class="doc_code"><pre> -MachinePassRegistry RegisterMyPasses::Registry; -</pre></div> - -<p>And finally, declare the command line option for your passes. Example:</p> - -<div class="doc_code"><pre> -cl::opt<RegisterMyPasses::FunctionPassCtor, false, - RegisterPassParser<RegisterMyPasses> > -MyPassOpt("mypass", - cl::init(&createDefaultMyPass), - cl::desc("my pass option help")); -</pre></div> - -<p>Here the command option is "mypass", with createDefaultMyPass as the default -creator.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="debughints">Using GDB with dynamically loaded passes</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it -should be. First of all, you can't set a breakpoint in a shared object that has -not been loaded yet, and second of all there are problems with inlined functions -in shared objects. Here are some suggestions to debugging your pass with -GDB.</p> - -<p>For sake of discussion, I'm going to assume that you are debugging a -transformation invoked by <tt>opt</tt>, although nothing described here depends -on that.</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="breakpoint">Setting a breakpoint in your pass</a> -</h4> - -<div> - -<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p> - -<div class="doc_code"><pre> -$ <b>gdb opt</b> -GNU gdb 5.0 -Copyright 2000 Free Software Foundation, Inc. -GDB is free software, covered by the GNU General Public License, and you are -welcome to change it and/or distribute copies of it under certain conditions. -Type "show copying" to see the conditions. -There is absolutely no warranty for GDB. Type "show warranty" for details. -This GDB was configured as "sparc-sun-solaris2.6"... -(gdb) -</pre></div> - -<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes -time to load. Be patient. Since we cannot set a breakpoint in our pass yet -(the shared object isn't loaded until runtime), we must execute the process, and -have it stop before it invokes our pass, but after it has loaded the shared -object. The most foolproof way of doing this is to set a breakpoint in -<tt>PassManager::run</tt> and then run the process with the arguments you -want:</p> - -<div class="doc_code"><pre> -(gdb) <b>break llvm::PassManager::run</b> -Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70. -(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]</b> -Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption] -Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70 -70 bool PassManager::run(Module &M) { return PM->run(M); } -(gdb) -</pre></div> - -<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are -now free to set breakpoints in your pass so that you can trace through execution -or do other standard debugging stuff.</p> - -</div> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="debugmisc">Miscellaneous Problems</a> -</h4> - -<div> - -<p>Once you have the basics down, there are a couple of problems that GDB has, -some with solutions, some without.</p> - -<ul> -<li>Inline functions have bogus stack information. In general, GDB does a -pretty good job getting stack traces and stepping through inline functions. -When a pass is dynamically loaded however, it somehow completely loses this -capability. The only solution I know of is to de-inline a function (move it -from the body of a class to a .cpp file).</li> - -<li>Restarting the program breaks breakpoints. After following the information -above, you have succeeded in getting some breakpoints planted in your pass. Nex -thing you know, you restart the program (i.e., you type '<tt>run</tt>' again), -and you start getting errors about breakpoints being unsettable. The only way I -have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are -already set in your pass, run the program, and re-set the breakpoints once -execution stops in <tt>PassManager::run</tt>.</li> - -</ul> - -<p>Hopefully these tips will help with common case debugging situations. If -you'd like to contribute some tips of your own, just contact <a -href="mailto:sabre@nondot.org">Chris</a>.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="future">Future extensions planned</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does -some nifty stuff, there are things we'd like to add in the future. Here is -where we are going:</p> - -<!-- _______________________________________________________________________ --> -<h4> - <a name="SMP">Multithreaded LLVM</a> -</h4> - -<div> - -<p>Multiple CPU machines are becoming more common and compilation can never be -fast enough: obviously we should allow for a multithreaded compiler. Because of -the semantics defined for passes above (specifically they cannot maintain state -across invocations of their <tt>run*</tt> methods), a nice clean way to -implement a multithreaded compiler would be for the <tt>PassManager</tt> class -to create multiple instances of each pass object, and allow the separate -instances to be hacking on different parts of the program at the same time.</p> - -<p>This implementation would prevent each of the passes from having to implement -multithreaded constructs, requiring only the LLVM core to have locking in a few -places (for global resources). Although this is a simple extension, we simply -haven't had time (or multiprocessor machines, thus a reason) to implement this. -Despite that, we have kept the LLVM passes SMP ready, and you should too.</p> - -</div> - -</div> - -<!-- *********************************************************************** --> -<hr> -<address> - <a href="http://jigsaw.w3.org/css-validator/check/referer"><img - src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> - <a href="http://validator.w3.org/check/referer"><img - src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> - - <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> - <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> - Last modified: $Date$ -</address> - -</body> -</html> diff --git a/docs/WritingAnLLVMPass.rst b/docs/WritingAnLLVMPass.rst new file mode 100644 index 0000000000..db47fefd93 --- /dev/null +++ b/docs/WritingAnLLVMPass.rst @@ -0,0 +1,1439 @@ +==================== +Writing an LLVM Pass +==================== + +.. contents:: + :local: + +Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and +`Jim Laskey <mailto:jlaskey@mac.com>`_ + +Introduction --- What is a pass? +================================ + +The LLVM Pass Framework is an important part of the LLVM system, because LLVM +passes are where most of the interesting parts of the compiler exist. Passes +perform the transformations and optimizations that make up the compiler, they +build the analysis results that are used by these transformations, and they +are, above all, a structuring technique for compiler code. + +All LLVM passes are subclasses of the `Pass +<http://llvm.org/doxygen/classllvm_1_1Pass.html>`_ class, which implement +functionality by overriding virtual methods inherited from ``Pass``. Depending +on how your pass works, you should inherit from the :ref:`ModulePass +<writing-an-llvm-pass-ModulePass>` , :ref:`CallGraphSCCPass +<writing-an-llvm-pass-CallGraphSCCPass>`, :ref:`FunctionPass +<writing-an-llvm-pass-FunctionPass>` , or :ref:`LoopPass +<writing-an-llvm-pass-LoopPass>`, or :ref:`RegionPass +<writing-an-llvm-pass-RegionPass>`, or :ref:`BasicBlockPass +<writing-an-llvm-pass-BasicBlockPass>` classes, which gives the system more +information about what your pass does, and how it can be combined with other +passes. One of the main features of the LLVM Pass Framework is that it +schedules passes to run in an efficient way based on the constraints that your +pass meets (which are indicated by which class they derive from). + +We start by showing you how to construct a pass, everything from setting up the +code, to compiling, loading, and executing it. After the basics are down, more +advanced features are discussed. + +Quick Start --- Writing hello world +=================================== + +Here we describe how to write the "hello world" of passes. The "Hello" pass is +designed to simply print out the name of non-external functions that exist in +the program being compiled. It does not modify the program at all, it just +inspects it. The source code and files for this pass are available in the LLVM +source tree in the ``lib/Transforms/Hello`` directory. + +.. _writing-an-llvm-pass-makefile: + +Setting up the build environment +-------------------------------- + +.. FIXME: Why does this recommend to build in-tree? + +First, configure and build LLVM. This needs to be done directly inside the +LLVM source tree rather than in a separate objects directory. Next, you need +to create a new directory somewhere in the LLVM source base. For this example, +we'll assume that you made ``lib/Transforms/Hello``. Finally, you must set up +a build script (``Makefile``) that will compile the source code for the new +pass. To do this, copy the following into ``Makefile``: + +.. code-block:: make + + # Makefile for hello pass + + # Path to top level of LLVM hierarchy + LEVEL = ../../.. + + # Name of the library to build + LIBRARYNAME = Hello + + # Make the shared library become a loadable module so the tools can + # dlopen/dlsym on the resulting library. + LOADABLE_MODULE = 1 + + # Include the makefile implementation stuff + include $(LEVEL)/Makefile.common + +This makefile specifies that all of the ``.cpp`` files in the current directory +are to be compiled and linked together into a shared object +``$(LEVEL)/Debug+Asserts/lib/Hello.so`` that can be dynamically loaded by the +:program:`opt` or :program:`bugpoint` tools via their :option:`-load` options. +If your operating system uses a suffix other than ``.so`` (such as Windows or Mac +OS X), the appropriate extension will be used. + +If you are used CMake to build LLVM, see :ref:`cmake-out-of-source-pass`. + +Now that we have the build scripts set up, we just need to write the code for +the pass itself. + +.. _writing-an-llvm-pass-basiccode: + +Basic code required +------------------- + +Now that we have a way to compile our new pass, we just have to write it. +Start out with: + +.. code-block:: c++ + + #include "llvm/Pass.h" + #include "llvm/Function.h" + #include "llvm/Support/raw_ostream.h" + +Which are needed because we are writing a `Pass +<http://llvm.org/doxygen/classllvm_1_1Pass.html>`_, we are operating on +`Function <http://llvm.org/doxygen/classllvm_1_1Function.html>`_\ s, and we will +be doing some printing. + +Next we have: + +.. code-block:: c++ + + using namespace llvm; + +... which is required because the functions from the include files live in the +llvm namespace. + +Next we have: + +.. code-block:: c++ + + namespace { + +... which starts out an anonymous namespace. Anonymous namespaces are to C++ +what the "``static``" keyword is to C (at global scope). It makes the things +declared inside of the anonymous namespace visible only to the current file. +If you're not familiar with them, consult a decent C++ book for more +information. + +Next, we declare our pass itself: + +.. code-block:: c++ + + struct Hello : public FunctionPass { + +This declares a "``Hello``" class that is a subclass of `FunctionPass +<writing-an-llvm-pass-FunctionPass>`. The different builtin pass subclasses +are described in detail :ref:`later <writing-an-llvm-pass-pass-classes>`, but +for now, know that ``FunctionPass`` operates on a function at a time. + +.. code-block:: c++ + + static char ID; + Hello() : FunctionPass(ID) {} + +This declares pass identifier used by LLVM to identify pass. This allows LLVM +to avoid using expensive C++ runtime information. + +.. code-block:: c++ + + virtual bool runOnFunction(Function &F) { + errs() << "Hello: "; + errs().write_escaped(F.getName()) << "\n"; + return false; + } + }; // end of struct Hello + } // end of anonymous namespace + +We declare a :ref:`runOnFunction <writing-an-llvm-pass-runOnFunction>` method, +which overrides an abstract virtual method inherited from :ref:`FunctionPass +<writing-an-llvm-pass-FunctionPass>`. This is where we are supposed to do our +thing, so we just print out our message with the name of each function. + +.. code-block:: c++ + + char Hello::ID = 0; + +We initialize pass ID here. LLVM uses ID's address to identify a pass, so +initialization value is not important. + +.. code-block:: c++ + + static RegisterPass<Hello> X("hello", "Hello World Pass", + false /* Only looks at CFG */, + false /* Analysis Pass */); + +Lastly, we :ref:`register our class <writing-an-llvm-pass-registration>` +``Hello``, giving it a command line argument "``hello``", and a name "Hello +World Pass". The last two arguments describe its behavior: if a pass walks CFG +without modifying it then the third argument is set to ``true``; if a pass is +an analysis pass, for example dominator tree pass, then ``true`` is supplied as +the fourth argument. + +As a whole, the ``.cpp`` file looks like: + +.. code-block:: c++ + + #include "llvm/Pass.h" + #include "llvm/Function.h" + #include "llvm/Support/raw_ostream.h" + + using namespace llvm; + + namespace { + struct Hello : public FunctionPass { + static char ID; + Hello() : FunctionPass(ID) {} + + virtual bool runOnFunction(Function &F) { + errs() << "Hello: "; + errs().write_escaped(F.getName()) << '\n'; + return false; + } + }; + } + + char Hello::ID = 0; + static RegisterPass<Hello> X("hello", "Hello World Pass", false, false); + +Now that it's all together, compile the file with a simple "``gmake``" command +in the local directory and you should get a new file +"``Debug+Asserts/lib/Hello.so``" under the top level directory of the LLVM +source tree (not in the local directory). Note that everything in this file is +contained in an anonymous namespace --- this reflects the fact that passes +are self contained units that do not need external interfaces (although they +can have them) to be useful. + +Running a pass with ``opt`` +--------------------------- + +Now that you have a brand new shiny shared object file, we can use the +:program:`opt` command to run an LLVM program through your pass. Because you +registered your pass with ``RegisterPass``, you will be able to use the +:program:`opt` tool to access it, once loaded. + +To test it, follow the example at the end of the :doc:`GettingStarted` to +compile "Hello World" to LLVM. We can now run the bitcode file (hello.bc) for +the program through our transformation like this (or course, any bitcode file +will work): + +.. code-block:: console + + $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null + Hello: __main + Hello: puts + Hello: main + +The :option:`-load` option specifies that :program:`opt` should load your pass +as a shared object, which makes "``-hello``" a valid command line argument +(which is one reason you need to :ref:`register your pass +<writing-an-llvm-pass-registration>`). Because the Hello pass does not modify +the program in any interesting way, we just throw away the result of +:program:`opt` (sending it to ``/dev/null``). + +To see what happened to the other string you registered, try running +:program:`opt` with the :option:`-help` option: + +.. code-block:: console + + $ opt -load ../../../Debug+Asserts/lib/Hello.so -help + OVERVIEW: llvm .bc -> .bc modular optimizer + + USAGE: opt [options] <input bitcode> + + OPTIONS: + Optimizations available: + ... + -globalopt - Global Variable Optimizer + -globalsmodref-aa - Simple mod/ref analysis for globals + -gvn - Global Value Numbering + -hello - Hello World Pass + -indvars - Induction Variable Simplification + -inline - Function Integration/Inlining + -insert-edge-profiling - Insert instrumentation for edge profiling + ... + +The pass name gets added as the information string for your pass, giving some +documentation to users of :program:`opt`. Now that you have a working pass, +you would go ahead and make it do the cool transformations you want. Once you +get it all working and tested, it may become useful to find out how fast your +pass is. The :ref:`PassManager <writing-an-llvm-pass-passmanager>` provides a +nice command line option (:option:`--time-passes`) that allows you to get +information about the execution time of your pass along with the other passes +you queue up. For example: + +.. code-block:: console + + $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null + Hello: __main + Hello: puts + Hello: main + =============================================================================== + ... Pass execution timing report ... + =============================================================================== + Total Execution Time: 0.02 seconds (0.0479059 wall clock) + + ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name --- + 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer + 0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction + 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier + 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass + 0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL + +As you can see, our implementation above is pretty fast. The additional +passes listed are automatically inserted by the :program:`opt` tool to verify +that the LLVM emitted by your pass is still valid and well formed LLVM, which +hasn't been broken somehow. + +Now that you have seen the basics of the mechanics behind passes, we can talk +about some more details of how they work and how to use them. + +.. _writing-an-llvm-pass-pass-classes: + +Pass classes and requirements +============================= + +One of the first things that you should do when designing a new pass is to +decide what class you should subclass for your pass. The :ref:`Hello World +<writing-an-llvm-pass-basiccode>` example uses the :ref:`FunctionPass +<writing-an-llvm-pass-FunctionPass>` class for its implementation, but we did +not discuss why or when this should occur. Here we talk about the classes +available, from the most general to the most specific. + +When choosing a superclass for your ``Pass``, you should choose the **most +specific** class possible, while still being able to meet the requirements +listed. This gives the LLVM Pass Infrastructure information necessary to +optimize how passes are run, so that the resultant compiler isn't unnecessarily +slow. + +The ``ImmutablePass`` class +--------------------------- + +The most plain and boring type of pass is the "`ImmutablePass +<http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html>`_" class. This pass +type is used for passes that do not have to be run, do not change state, and +never need to be updated. This is not a normal type of transformation or +analysis, but can provide information about the current compiler configuration. + +Although this pass class is very infrequently used, it is important for +providing information about the current target machine being compiled for, and +other static information that can affect the various transformations. + +``ImmutablePass``\ es never invalidate other transformations, are never +invalidated, and are never "run". + +.. _writing-an-llvm-pass-ModulePass: + +The ``ModulePass`` class +------------------------ + +The `ModulePass <http://llvm.org/doxygen/classllvm_1_1ModulePass.html>`_ class +is the most general of all superclasses that you can use. Deriving from +``ModulePass`` indicates that your pass uses the entire program as a unit, +referring to function bodies in no predictable order, or adding and removing +functions. Because nothing is known about the behavior of ``ModulePass`` +subclasses, no optimization can be done for their execution. + +A module pass can use function level passes (e.g. dominators) using the +``getAnalysis`` interface ``getAnalysis<DominatorTree>(llvm::Function *)`` to +provide the function to retrieve analysis result for, if the function pass does +not require any module or immutable passes. Note that this can only be done +for functions for which the analysis ran, e.g. in the case of dominators you +should only ask for the ``DominatorTree`` for function definitions, not +declarations. + +To write a correct ``ModulePass`` subclass, derive from ``ModulePass`` and +overload the ``runOnModule`` method with the following signature: + +The ``runOnModule`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool runOnModule(Module &M) = 0; + +The ``runOnModule`` method performs the interesting work of the pass. It +should return ``true`` if the module was modified by the transformation and +``false`` otherwise. + +.. _writing-an-llvm-pass-CallGraphSCCPass: + +The ``CallGraphSCCPass`` class +------------------------------ + +The `CallGraphSCCPass +<http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html>`_ is used by +passes that need to traverse the program bottom-up on the call graph (callees +before callers). Deriving from ``CallGraphSCCPass`` provides some mechanics +for building and traversing the ``CallGraph``, but also allows the system to +optimize execution of ``CallGraphSCCPass``\ es. If your pass meets the +requirements outlined below, and doesn't meet the requirements of a +:ref:`FunctionPass <writing-an-llvm-pass-FunctionPass>` or :ref:`BasicBlockPass +<writing-an-llvm-pass-BasicBlockPass>`, you should derive from +``CallGraphSCCPass``. + +``TODO``: explain briefly what SCC, Tarjan's algo, and B-U mean. + +To be explicit, CallGraphSCCPass subclasses are: + +#. ... *not allowed* to inspect or modify any ``Function``\ s other than those + in the current SCC and the direct callers and direct callees of the SCC. +#. ... *required* to preserve the current ``CallGraph`` object, updating it to + reflect any changes made to the program. +#. ... *not allowed* to add or remove SCC's from the current Module, though + they may change the contents of an SCC. +#. ... *allowed* to add or remove global variables from the current Module. +#. ... *allowed* to maintain state across invocations of :ref:`runOnSCC + <writing-an-llvm-pass-runOnSCC>` (including global data). + +Implementing a ``CallGraphSCCPass`` is slightly tricky in some cases because it +has to handle SCCs with more than one node in it. All of the virtual methods +described below should return ``true`` if they modified the program, or +``false`` if they didn't. + +The ``doInitialization(CallGraph &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doInitialization(CallGraph &CG); + +The ``doInitialization`` method is allowed to do most of the things that +``CallGraphSCCPass``\ es are not allowed to do. They can add and remove +functions, get pointers to functions, etc. The ``doInitialization`` method is +designed to do simple initialization type of stuff that does not depend on the +SCCs being processed. The ``doInitialization`` method call is not scheduled to +overlap with any other pass executions (thus it should be very fast). + +.. _writing-an-llvm-pass-runOnSCC: + +The ``runOnSCC`` method +^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool runOnSCC(CallGraphSCC &SCC) = 0; + +The ``runOnSCC`` method performs the interesting work of the pass, and should +return ``true`` if the module was modified by the transformation, ``false`` +otherwise. + +The ``doFinalization(CallGraph &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doFinalization(CallGraph &CG); + +The ``doFinalization`` method is an infrequently used method that is called +when the pass framework has finished calling :ref:`runOnFunction +<writing-an-llvm-pass-runOnFunction>` for every function in the program being +compiled. + +.. _writing-an-llvm-pass-FunctionPass: + +The ``FunctionPass`` class +-------------------------- + +In contrast to ``ModulePass`` subclasses, `FunctionPass +<http://llvm.org/doxygen/classllvm_1_1Pass.html>`_ subclasses do have a +predictable, local behavior that can be expected by the system. All +``FunctionPass`` execute on each function in the program independent of all of +the other functions in the program. ``FunctionPass``\ es do not require that +they are executed in a particular order, and ``FunctionPass``\ es do not modify +external functions. + +To be explicit, ``FunctionPass`` subclasses are not allowed to: + +#. Modify a ``Function`` other than the one currently being processed. +#. Add or remove ``Function``\ s from the current ``Module``. +#. Add or remove global variables from the current ``Module``. +#. Maintain state across invocations of:ref:`runOnFunction + <writing-an-llvm-pass-runOnFunction>` (including global data). + +Implementing a ``FunctionPass`` is usually straightforward (See the :ref:`Hello +World <writing-an-llvm-pass-basiccode>` pass for example). +``FunctionPass``\ es may overload three virtual methods to do their work. All +of these methods should return ``true`` if they modified the program, or +``false`` if they didn't. + +.. _writing-an-llvm-pass-doInitialization-mod: + +The ``doInitialization(Module &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doInitialization(Module &M); + +The ``doInitialization`` method is allowed to do most of the things that +``FunctionPass``\ es are not allowed to do. They can add and remove functions, +get pointers to functions, etc. The ``doInitialization`` method is designed to +do simple initialization type of stuff that does not depend on the functions +being processed. The ``doInitialization`` method call is not scheduled to +overlap with any other pass executions (thus it should be very fast). + +A good example of how this method should be used is the `LowerAllocations +<http://llvm.org/doxygen/LowerAllocations_8cpp-source.html>`_ pass. This pass +converts ``malloc`` and ``free`` instructions into platform dependent +``malloc()`` and ``free()`` function calls. It uses the ``doInitialization`` +method to get a reference to the ``malloc`` and ``free`` functions that it +needs, adding prototypes to the module if necessary. + +.. _writing-an-llvm-pass-runOnFunction: + +The ``runOnFunction`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool runOnFunction(Function &F) = 0; + +The ``runOnFunction`` method must be implemented by your subclass to do the +transformation or analysis work of your pass. As usual, a ``true`` value +should be returned if the function is modified. + +.. _writing-an-llvm-pass-doFinalization-mod: + +The ``doFinalization(Module &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doFinalization(Module &M); + +The ``doFinalization`` method is an infrequently used method that is called +when the pass framework has finished calling :ref:`runOnFunction +<writing-an-llvm-pass-runOnFunction>` for every function in the program being +compiled. + +.. _writing-an-llvm-pass-LoopPass: + +The ``LoopPass`` class +---------------------- + +All ``LoopPass`` execute on each loop in the function independent of all of the +other loops in the function. ``LoopPass`` processes loops in loop nest order +such that outer most loop is processed last. + +``LoopPass`` subclasses are allowed to update loop nest using ``LPPassManager`` +interface. Implementing a loop pass is usually straightforward. +``LoopPass``\ es may overload three virtual methods to do their work. All +these methods should return ``true`` if they modified the program, or ``false`` +if they didn't. + +The ``doInitialization(Loop *, LPPassManager &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doInitialization(Loop *, LPPassManager &LPM); + +The ``doInitialization`` method is designed to do simple initialization type of +stuff that does not depend on the functions being processed. The +``doInitialization`` method call is not scheduled to overlap with any other +pass executions (thus it should be very fast). ``LPPassManager`` interface +should be used to access ``Function`` or ``Module`` level analysis information. + +.. _writing-an-llvm-pass-runOnLoop: + +The ``runOnLoop`` method +^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool runOnLoop(Loop *, LPPassManager &LPM) = 0; + +The ``runOnLoop`` method must be implemented by your subclass to do the +transformation or analysis work of your pass. As usual, a ``true`` value +should be returned if the function is modified. ``LPPassManager`` interface +should be used to update loop nest. + +The ``doFinalization()`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doFinalization(); + +The ``doFinalization`` method is an infrequently used method that is called +when the pass framework has finished calling :ref:`runOnLoop +<writing-an-llvm-pass-runOnLoop>` for every loop in the program being compiled. + +.. _writing-an-llvm-pass-RegionPass: + +The ``RegionPass`` class +------------------------ + +``RegionPass`` is similar to :ref:`LoopPass <writing-an-llvm-pass-LoopPass>`, +but executes on each single entry single exit region in the function. +``RegionPass`` processes regions in nested order such that the outer most +region is processed last. + +``RegionPass`` subclasses are allowed to update the region tree by using the +``RGPassManager`` interface. You may overload three virtual methods of +``RegionPass`` to implement your own region pass. All these methods should +return ``true`` if they modified the program, or ``false`` if they did not. + +The ``doInitialization(Region *, RGPassManager &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doInitialization(Region *, RGPassManager &RGM); + +The ``doInitialization`` method is designed to do simple initialization type of +stuff that does not depend on the functions being processed. The +``doInitialization`` method call is not scheduled to overlap with any other +pass executions (thus it should be very fast). ``RPPassManager`` interface +should be used to access ``Function`` or ``Module`` level analysis information. + +.. _writing-an-llvm-pass-runOnRegion: + +The ``runOnRegion`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool runOnRegion(Region *, RGPassManager &RGM) = 0; + +The ``runOnRegion`` method must be implemented by your subclass to do the +transformation or analysis work of your pass. As usual, a true value should be +returned if the region is modified. ``RGPassManager`` interface should be used to +update region tree. + +The ``doFinalization()`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doFinalization(); + +The ``doFinalization`` method is an infrequently used method that is called +when the pass framework has finished calling :ref:`runOnRegion +<writing-an-llvm-pass-runOnRegion>` for every region in the program being +compiled. + +.. _writing-an-llvm-pass-BasicBlockPass: + +The ``BasicBlockPass`` class +---------------------------- + +``BasicBlockPass``\ es are just like :ref:`FunctionPass's +<writing-an-llvm-pass-FunctionPass>` , except that they must limit their scope +of inspection and modification to a single basic block at a time. As such, +they are **not** allowed to do any of the following: + +#. Modify or inspect any basic blocks outside of the current one. +#. Maintain state across invocations of :ref:`runOnBasicBlock + <writing-an-llvm-pass-runOnBasicBlock>`. +#. Modify the control flow graph (by altering terminator instructions) +#. Any of the things forbidden for :ref:`FunctionPasses + <writing-an-llvm-pass-FunctionPass>`. + +``BasicBlockPass``\ es are useful for traditional local and "peephole" +optimizations. They may override the same :ref:`doInitialization(Module &) +<writing-an-llvm-pass-doInitialization-mod>` and :ref:`doFinalization(Module &) +<writing-an-llvm-pass-doFinalization-mod>` methods that :ref:`FunctionPass's +<writing-an-llvm-pass-FunctionPass>` have, but also have the following virtual +methods that may also be implemented: + +The ``doInitialization(Function &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doInitialization(Function &F); + +The ``doInitialization`` method is allowed to do most of the things that +``BasicBlockPass``\ es are not allowed to do, but that ``FunctionPass``\ es +can. The ``doInitialization`` method is designed to do simple initialization +that does not depend on the ``BasicBlock``\ s being processed. The +``doInitialization`` method call is not scheduled to overlap with any other +pass executions (thus it should be very fast). + +.. _writing-an-llvm-pass-runOnBasicBlock: + +The ``runOnBasicBlock`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool runOnBasicBlock(BasicBlock &BB) = 0; + +Override this function to do the work of the ``BasicBlockPass``. This function +is not allowed to inspect or modify basic blocks other than the parameter, and +are not allowed to modify the CFG. A ``true`` value must be returned if the +basic block is modified. + +The ``doFinalization(Function &)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool doFinalization(Function &F); + +The ``doFinalization`` method is an infrequently used method that is called +when the pass framework has finished calling :ref:`runOnBasicBlock +<writing-an-llvm-pass-runOnBasicBlock>` for every ``BasicBlock`` in the program +being compiled. This can be used to perform per-function finalization. + +The ``MachineFunctionPass`` class +--------------------------------- + +A ``MachineFunctionPass`` is a part of the LLVM code generator that executes on +the machine-dependent representation of each LLVM function in the program. + +Code generator passes are registered and initialized specially by +``TargetMachine::addPassesToEmitFile`` and similar routines, so they cannot +generally be run from the :program:`opt` or :program:`bugpoint` commands. + +A ``MachineFunctionPass`` is also a ``FunctionPass``, so all the restrictions +that apply to a ``FunctionPass`` also apply to it. ``MachineFunctionPass``\ es +also have additional restrictions. In particular, ``MachineFunctionPass``\ es +are not allowed to do any of the following: + +#. Modify or create any LLVM IR ``Instruction``\ s, ``BasicBlock``\ s, + ``Argument``\ s, ``Function``\ s, ``GlobalVariable``\ s, + ``GlobalAlias``\ es, or ``Module``\ s. +#. Modify a ``MachineFunction`` other than the one currently being processed. +#. Maintain state across invocations of :ref:`runOnMachineFunction + <writing-an-llvm-pass-runOnMachineFunction>` (including global data). + +.. _writing-an-llvm-pass-runOnMachineFunction: + +The ``runOnMachineFunction(MachineFunction &MF)`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual bool runOnMachineFunction(MachineFunction &MF) = 0; + +``runOnMachineFunction`` can be considered the main entry point of a +``MachineFunctionPass``; that is, you should override this method to do the +work of your ``MachineFunctionPass``. + +The ``runOnMachineFunction`` method is called on every ``MachineFunction`` in a +``Module``, so that the ``MachineFunctionPass`` may perform optimizations on +the machine-dependent representation of the function. If you want to get at +the LLVM ``Function`` for the ``MachineFunction`` you're working on, use +``MachineFunction``'s ``getFunction()`` accessor method --- but remember, you +may not modify the LLVM ``Function`` or its contents from a +``MachineFunctionPass``. + +.. _writing-an-llvm-pass-registration: + +Pass registration +----------------- + +In the :ref:`Hello World <writing-an-llvm-pass-basiccode>` example pass we +illustrated how pass registration works, and discussed some of the reasons that +it is used and what it does. Here we discuss how and why passes are +registered. + +As we saw above, passes are registered with the ``RegisterPass`` template. The +template parameter is the name of the pass that is to be used on the command +line to specify that the pass should be added to a program (for example, with +:program:`opt` or :program:`bugpoint`). The first argument is the name of the +pass, which is to be used for the :option:`-help` output of programs, as well +as for debug output generated by the :option:`--debug-pass` option. + +If you want your pass to be easily dumpable, you should implement the virtual +print method: + +The ``print`` method +^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual void print(llvm::raw_ostream &O, const Module *M) const; + +The ``print`` method must be implemented by "analyses" in order to print a +human readable version of the analysis results. This is useful for debugging +an analysis itself, as well as for other people to figure out how an analysis +works. Use the opt ``-analyze`` argument to invoke this method. + +The ``llvm::raw_ostream`` parameter specifies the stream to write the results +on, and the ``Module`` parameter gives a pointer to the top level module of the +program that has been analyzed. Note however that this pointer may be ``NULL`` +in certain circumstances (such as calling the ``Pass::dump()`` from a +debugger), so it should only be used to enhance debug output, it should not be +depended on. + +.. _writing-an-llvm-pass-interaction: + +Specifying interactions between passes +-------------------------------------- + +One of the main responsibilities of the ``PassManager`` is to make sure that +passes interact with each other correctly. Because ``PassManager`` tries to +:ref:`optimize the execution of passes <writing-an-llvm-pass-passmanager>` it +must know how the passes interact with each other and what dependencies exist +between the various passes. To track this, each pass can declare the set of +passes that are required to be executed before the current pass, and the passes +which are invalidated by the current pass. + +Typically this functionality is used to require that analysis results are +computed before your pass is run. Running arbitrary transformation passes can +invalidate the computed analysis results, which is what the invalidation set +specifies. If a pass does not implement the :ref:`getAnalysisUsage +<writing-an-llvm-pass-getAnalysisUsage>` method, it defaults to not having any +prerequisite passes, and invalidating **all** other passes. + +.. _writing-an-llvm-pass-getAnalysisUsage: + +The ``getAnalysisUsage`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual void getAnalysisUsage(AnalysisUsage &Info) const; + +By implementing the ``getAnalysisUsage`` method, the required and invalidated +sets may be specified for your transformation. The implementation should fill +in the `AnalysisUsage +<http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html>`_ object with +information about which passes are required and not invalidated. To do this, a +pass may call any of the following methods on the ``AnalysisUsage`` object: + +The ``AnalysisUsage::addRequired<>`` and ``AnalysisUsage::addRequiredTransitive<>`` methods +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +If your pass requires a previous pass to be executed (an analysis for example), +it can use one of these methods to arrange for it to be run before your pass. +LLVM has many different types of analyses and passes that can be required, +spanning the range from ``DominatorSet`` to ``BreakCriticalEdges``. Requiring +``BreakCriticalEdges``, for example, guarantees that there will be no critical +edges in the CFG when your pass has been run. + +Some analyses chain to other analyses to do their job. For example, an +`AliasAnalysis <AliasAnalysis>` implementation is required to :ref:`chain +<aliasanalysis-chaining>` to other alias analysis passes. In cases where +analyses chain, the ``addRequiredTransitive`` method should be used instead of +the ``addRequired`` method. This informs the ``PassManager`` that the +transitively required pass should be alive as long as the requiring pass is. + +The ``AnalysisUsage::addPreserved<>`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +One of the jobs of the ``PassManager`` is to optimize how and when analyses are +run. In particular, it attempts to avoid recomputing data unless it needs to. +For this reason, passes are allowed to declare that they preserve (i.e., they +don't invalidate) an existing analysis if it's available. For example, a +simple constant folding pass would not modify the CFG, so it can't possibly +affect the results of dominator analysis. By default, all passes are assumed +to invalidate all others. + +The ``AnalysisUsage`` class provides several methods which are useful in +certain circumstances that are related to ``addPreserved``. In particular, the +``setPreservesAll`` method can be called to indicate that the pass does not +modify the LLVM program at all (which is true for analyses), and the +``setPreservesCFG`` method can be used by transformations that change +instructions in the program but do not modify the CFG or terminator +instructions (note that this property is implicitly set for +:ref:`BasicBlockPass <writing-an-llvm-pass-BasicBlockPass>`\ es). + +``addPreserved`` is particularly useful for transformations like +``BreakCriticalEdges``. This pass knows how to update a small set of loop and +dominator related analyses if they exist, so it can preserve them, despite the +fact that it hacks on the CFG. + +Example implementations of ``getAnalysisUsage`` +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + // This example modifies the program, but does not modify the CFG + void LICM::getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesCFG(); + AU.addRequired<LoopInfo>(); + } + +.. _writing-an-llvm-pass-getAnalysis: + +The ``getAnalysis<>`` and ``getAnalysisIfAvailable<>`` methods +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The ``Pass::getAnalysis<>`` method is automatically inherited by your class, +providing you with access to the passes that you declared that you required +with the :ref:`getAnalysisUsage <writing-an-llvm-pass-getAnalysisUsage>` +method. It takes a single template argument that specifies which pass class +you want, and returns a reference to that pass. For example: + +.. code-block:: c++ + + bool LICM::runOnFunction(Function &F) { + LoopInfo &LI = getAnalysis<LoopInfo>(); + //... + } + +This method call returns a reference to the pass desired. You may get a +runtime assertion failure if you attempt to get an analysis that you did not +declare as required in your :ref:`getAnalysisUsage +<writing-an-llvm-pass-getAnalysisUsage>` implementation. This method can be +called by your ``run*`` method implementation, or by any other local method +invoked by your ``run*`` method. + +A module level pass can use function level analysis info using this interface. +For example: + +.. code-block:: c++ + + bool ModuleLevelPass::runOnModule(Module &M) { + //... + DominatorTree &DT = getAnalysis<DominatorTree>(Func); + //... + } + +In above example, ``runOnFunction`` for ``DominatorTree`` is called by pass +manager before returning a reference to the desired pass. + +If your pass is capable of updating analyses if they exist (e.g., +``BreakCriticalEdges``, as described above), you can use the +``getAnalysisIfAvailable`` method, which returns a pointer to the analysis if +it is active. For example: + +.. code-block:: c++ + + if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) { + // A DominatorSet is active. This code will update it. + } + +Implementing Analysis Groups +---------------------------- + +Now that we understand the basics of how passes are defined, how they are used, +and how they are required from other passes, it's time to get a little bit +fancier. All of the pass relationships that we have seen so far are very +simple: one pass depends on one other specific pass to be run before it can +run. For many applications, this is great, for others, more flexibility is +required. + +In particular, some analyses are defined such that there is a single simple +interface to the analysis results, but multiple ways of calculating them. +Consider alias analysis for example. The most trivial alias analysis returns +"may alias" for any alias query. The most sophisticated analysis a +flow-sensitive, context-sensitive interprocedural analysis that can take a +significant amount of time to execute (and obviously, there is a lot of room +between these two extremes for other implementations). To cleanly support +situations like this, the LLVM Pass Infrastructure supports the notion of +Analysis Groups. + +Analysis Group Concepts +^^^^^^^^^^^^^^^^^^^^^^^ + +An Analysis Group is a single simple interface that may be implemented by +multiple different passes. Analysis Groups can be given human readable names +just like passes, but unlike passes, they need not derive from the ``Pass`` +class. An analysis group may have one or more implementations, one of which is +the "default" implementation. + +Analysis groups are used by client passes just like other passes are: the +``AnalysisUsage::addRequired()`` and ``Pass::getAnalysis()`` methods. In order +to resolve this requirement, the :ref:`PassManager +<writing-an-llvm-pass-passmanager>` scans the available passes to see if any +implementations of the analysis group are available. If none is available, the +default implementation is created for the pass to use. All standard rules for +:ref:`interaction between passes <writing-an-llvm-pass-interaction>` still +apply. + +Although :ref:`Pass Registration <writing-an-llvm-pass-registration>` is +optional for normal passes, all analysis group implementations must be +registered, and must use the :ref:`INITIALIZE_AG_PASS +<writing-an-llvm-pass-RegisterAnalysisGroup>` template to join the +implementation pool. Also, a default implementation of the interface **must** +be registered with :ref:`RegisterAnalysisGroup +<writing-an-llvm-pass-RegisterAnalysisGroup>`. + +As a concrete example of an Analysis Group in action, consider the +`AliasAnalysis <http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`_ +analysis group. The default implementation of the alias analysis interface +(the `basicaa <http://llvm.org/doxygen/structBasicAliasAnalysis.html>`_ pass) +just does a few simple checks that don't require significant analysis to +compute (such as: two different globals can never alias each other, etc). +Passes that use the `AliasAnalysis +<http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`_ interface (for +example the `gcse <http://llvm.org/doxygen/structGCSE.html>`_ pass), do not +care which implementation of alias analysis is actually provided, they just use +the designated interface. + +From the user's perspective, commands work just like normal. Issuing the +command ``opt -gcse ...`` will cause the ``basicaa`` class to be instantiated +and added to the pass sequence. Issuing the command ``opt -somefancyaa -gcse +...`` will cause the ``gcse`` pass to use the ``somefancyaa`` alias analysis +(which doesn't actually exist, it's just a hypothetical example) instead. + +.. _writing-an-llvm-pass-RegisterAnalysisGroup: + +Using ``RegisterAnalysisGroup`` +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The ``RegisterAnalysisGroup`` template is used to register the analysis group +itself, while the ``INITIALIZE_AG_PASS`` is used to add pass implementations to +the analysis group. First, an analysis group should be registered, with a +human readable name provided for it. Unlike registration of passes, there is +no command line argument to be specified for the Analysis Group Interface +itself, because it is "abstract": + +.. code-block:: c++ + + static RegisterAnalysisGroup<AliasAnalysis> A("Alias Analysis"); + +Once the analysis is registered, passes can declare that they are valid +implementations of the interface by using the following code: + +.. code-block:: c++ + + namespace { + // Declare that we implement the AliasAnalysis interface + INITIALIZE_AG_PASS(FancyAA, AliasAnalysis , "somefancyaa", + "A more complex alias analysis implementation", + false, // Is CFG Only? + true, // Is Analysis? + false); // Is default Analysis Group implementation? + } + +This just shows a class ``FancyAA`` that uses the ``INITIALIZE_AG_PASS`` macro +both to register and to "join" the `AliasAnalysis +<http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`_ analysis group. +Every implementation of an analysis group should join using this macro. + +.. code-block:: c++ + + namespace { + // Declare that we implement the AliasAnalysis interface + INITIALIZE_AG_PASS(BasicAA, AliasAnalysis, "basicaa", + "Basic Alias Analysis (default AA impl)", + false, // Is CFG Only? + true, // Is Analysis? + true); // Is default Analysis Group implementation? + } + +Here we show how the default implementation is specified (using the final +argument to the ``INITIALIZE_AG_PASS`` template). There must be exactly one +default implementation available at all times for an Analysis Group to be used. +Only default implementation can derive from ``ImmutablePass``. Here we declare +that the `BasicAliasAnalysis +<http://llvm.org/doxygen/structBasicAliasAnalysis.html>`_ pass is the default +implementation for the interface. + +Pass Statistics +=============== + +The `Statistic <http://llvm.org/doxygen/Statistic_8h-source.html>`_ class is +designed to be an easy way to expose various success metrics from passes. +These statistics are printed at the end of a run, when the :option:`-stats` +command line option is enabled on the command line. See the :ref:`Statistics +section <Statistic>` in the Programmer's Manual for details. + +.. _writing-an-llvm-pass-passmanager: + +What PassManager does +--------------------- + +The `PassManager <http://llvm.org/doxygen/PassManager_8h-source.html>`_ `class +<http://llvm.org/doxygen/classllvm_1_1PassManager.html>`_ takes a list of +passes, ensures their :ref:`prerequisites <writing-an-llvm-pass-interaction>` +are set up correctly, and then schedules passes to run efficiently. All of the +LLVM tools that run passes use the PassManager for execution of these passes. + +The PassManager does two main things to try to reduce the execution time of a +series of passes: + +#. **Share analysis results.** The ``PassManager`` attempts to avoid + recomputing analysis results as much as possible. This means keeping track + of which analyses are available already, which analyses get invalidated, and + which analyses are needed to be run for a pass. An important part of work + is that the ``PassManager`` tracks the exact lifetime of all analysis + results, allowing it to :ref:`free memory + <writing-an-llvm-pass-releaseMemory>` allocated to holding analysis results + as soon as they are no longer needed. + +#. **Pipeline the execution of passes on the program.** The ``PassManager`` + attempts to get better cache and memory usage behavior out of a series of + passes by pipelining the passes together. This means that, given a series + of consecutive :ref:`FunctionPass <writing-an-llvm-pass-FunctionPass>`, it + will execute all of the :ref:`FunctionPass + <writing-an-llvm-pass-FunctionPass>` on the first function, then all of the + :ref:`FunctionPasses <writing-an-llvm-pass-FunctionPass>` on the second + function, etc... until the entire program has been run through the passes. + + This improves the cache behavior of the compiler, because it is only + touching the LLVM program representation for a single function at a time, + instead of traversing the entire program. It reduces the memory consumption + of compiler, because, for example, only one `DominatorSet + <http://llvm.org/doxygen/classllvm_1_1DominatorSet.html>`_ needs to be + calculated at a time. This also makes it possible to implement some + :ref:`interesting enhancements <writing-an-llvm-pass-SMP>` in the future. + +The effectiveness of the ``PassManager`` is influenced directly by how much +information it has about the behaviors of the passes it is scheduling. For +example, the "preserved" set is intentionally conservative in the face of an +unimplemented :ref:`getAnalysisUsage <writing-an-llvm-pass-getAnalysisUsage>` +method. Not implementing when it should be implemented will have the effect of +not allowing any analysis results to live across the execution of your pass. + +The ``PassManager`` class exposes a ``--debug-pass`` command line options that +is useful for debugging pass execution, seeing how things work, and diagnosing +when you should be preserving more analyses than you currently are. (To get +information about all of the variants of the ``--debug-pass`` option, just type +"``opt -help-hidden``"). + +By using the --debug-pass=Structure option, for example, we can see how our +:ref:`Hello World <writing-an-llvm-pass-basiccode>` pass interacts with other +passes. Lets try it out with the gcse and licm passes: + +.. code-block:: console + + $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null + Module Pass Manager + Function Pass Manager + Dominator Set Construction + Immediate Dominators Construction + Global Common Subexpression Elimination + -- Immediate Dominators Construction + -- Global Common Subexpression Elimination + Natural Loop Construction + Loop Invariant Code Motion + -- Natural Loop Construction + -- Loop Invariant Code Motion + Module Verifier + -- Dominator Set Construction + -- Module Verifier + Bitcode Writer + --Bitcode Writer + +This output shows us when passes are constructed and when the analysis results +are known to be dead (prefixed with "``--``"). Here we see that GCSE uses +dominator and immediate dominator information to do its job. The LICM pass +uses natural loop information, which uses dominator sets, but not immediate +dominators. Because immediate dominators are no longer useful after the GCSE +pass, it is immediately destroyed. The dominator sets are then reused to +compute natural loop information, which is then used by the LICM pass. + +After the LICM pass, the module verifier runs (which is automatically added by +the :program:`opt` tool), which uses the dominator set to check that the +resultant LLVM code is well formed. After it finishes, the dominator set +information is destroyed, after being computed once, and shared by three +passes. + +Lets see how this changes when we run the :ref:`Hello World +<writing-an-llvm-pass-basiccode>` pass in between the two passes: + +.. code-block:: console + + $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null + Module Pass Manager + Function Pass Manager + Dominator Set Construction + Immediate Dominators Construction + Global Common Subexpression Elimination + -- Dominator Set Construction + -- Immediate Dominators Construction + -- Global Common Subexpression Elimination + Hello World Pass + -- Hello World Pass + Dominator Set Construction + Natural Loop Construction + Loop Invariant Code Motion + -- Natural Loop Construction + -- Loop Invariant Code Motion + Module Verifier + -- Dominator Set Construction + -- Module Verifier + Bitcode Writer + --Bitcode Writer + Hello: __main + Hello: puts + Hello: main + +Here we see that the :ref:`Hello World <writing-an-llvm-pass-basiccode>` pass +has killed the Dominator Set pass, even though it doesn't modify the code at +all! To fix this, we need to add the following :ref:`getAnalysisUsage +<writing-an-llvm-pass-getAnalysisUsage>` method to our pass: + +.. code-block:: c++ + + // We don't modify the program, so we preserve all analyses + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesAll(); + } + +Now when we run our pass, we get this output: + +.. code-block:: console + + $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null + Pass Arguments: -gcse -hello -licm + Module Pass Manager + Function Pass Manager + Dominator Set Construction + Immediate Dominators Construction + Global Common Subexpression Elimination + -- Immediate Dominators Construction + -- Global Common Subexpression Elimination + Hello World Pass + -- Hello World Pass + Natural Loop Construction + Loop Invariant Code Motion + -- Loop Invariant Code Motion + -- Natural Loop Construction + Module Verifier + -- Dominator Set Construction + -- Module Verifier + Bitcode Writer + --Bitcode Writer + Hello: __main + Hello: puts + Hello: main + +Which shows that we don't accidentally invalidate dominator information +anymore, and therefore do not have to compute it twice. + +.. _writing-an-llvm-pass-releaseMemory: + +The ``releaseMemory`` method +^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +.. code-block:: c++ + + virtual void releaseMemory(); + +The ``PassManager`` automatically determines when to compute analysis results, +and how long to keep them around for. Because the lifetime of the pass object +itself is effectively the entire duration of the compilation process, we need +some way to free analysis results when they are no longer useful. The +``releaseMemory`` virtual method is the way to do this. + +If you are writing an analysis or any other pass that retains a significant +amount of state (for use by another pass which "requires" your pass and uses +the :ref:`getAnalysis <writing-an-llvm-pass-getAnalysis>` method) you should +implement ``releaseMemory`` to, well, release the memory allocated to maintain +this internal state. This method is called after the ``run*`` method for the +class, before the next call of ``run*`` in your pass. + +Registering dynamically loaded passes +===================================== + +*Size matters* when constructing production quality tools using LLVM, both for +the purposes of distribution, and for regulating the resident code size when +running on the target system. Therefore, it becomes desirable to selectively +use some passes, while omitting others and maintain the flexibility to change +configurations later on. You want to be able to do all this, and, provide +feedback to the user. This is where pass registration comes into play. + +The fundamental mechanisms for pass registration are the +``MachinePassRegistry`` class and subclasses of ``MachinePassRegistryNode``. + +An instance of ``MachinePassRegistry`` is used to maintain a list of +``MachinePassRegistryNode`` objects. This instance maintains the list and +communicates additions and deletions to the command line interface. + +An instance of ``MachinePassRegistryNode`` subclass is used to maintain +information provided about a particular pass. This information includes the +command line name, the command help string and the address of the function used +to create an instance of the pass. A global static constructor of one of these +instances *registers* with a corresponding ``MachinePassRegistry``, the static +destructor *unregisters*. Thus a pass that is statically linked in the tool +will be registered at start up. A dynamically loaded pass will register on +load and unregister at unload. + +Using existing registries +------------------------- + +There are predefined registries to track instruction scheduling +(``RegisterScheduler``) and register allocation (``RegisterRegAlloc``) machine +passes. Here we will describe how to *register* a register allocator machine +pass. + +Implement your register allocator machine pass. In your register allocator +``.cpp`` file add the following include: + +.. code-block:: c++ + + #include "llvm/CodeGen/RegAllocRegistry.h" + +Also in your register allocator ``.cpp`` file, define a creator function in the +form: + +.. code-block:: c++ + + FunctionPass *createMyRegisterAllocator() { + return new MyRegisterAllocator(); + } + +Note that the signature of this function should match the type of +``RegisterRegAlloc::FunctionPassCtor``. In the same file add the "installing" +declaration, in the form: + +.. code-block:: c++ + + static RegisterRegAlloc myRegAlloc("myregalloc", + "my register allocator help string", + createMyRegisterAllocator); + +Note the two spaces prior to the help string produces a tidy result on the +:option:`-help` query. + +.. code-block:: console + + $ llc -help + ... + -regalloc - Register allocator to use (default=linearscan) + =linearscan - linear scan register allocator + =local - local register allocator + =simple - simple register allocator + =myregalloc - my register allocator help string + ... + +And that's it. The user is now free to use ``-regalloc=myregalloc`` as an +option. Registering instruction schedulers is similar except use the +``RegisterScheduler`` class. Note that the +``RegisterScheduler::FunctionPassCtor`` is significantly different from +``RegisterRegAlloc::FunctionPassCtor``. + +To force the load/linking of your register allocator into the +:program:`llc`/:program:`lli` tools, add your creator function's global +declaration to ``Passes.h`` and add a "pseudo" call line to +``llvm/Codegen/LinkAllCodegenComponents.h``. + +Creating new registries +----------------------- + +The easiest way to get started is to clone one of the existing registries; we +recommend ``llvm/CodeGen/RegAllocRegistry.h``. The key things to modify are +the class name and the ``FunctionPassCtor`` type. + +Then you need to declare the registry. Example: if your pass registry is +``RegisterMyPasses`` then define: + +.. code-block:: c++ + + MachinePassRegistry RegisterMyPasses::Registry; + +And finally, declare the command line option for your passes. Example: + +.. code-block:: c++ + + cl::opt<RegisterMyPasses::FunctionPassCtor, false, + RegisterPassParser<RegisterMyPasses> > + MyPassOpt("mypass", + cl::init(&createDefaultMyPass), + cl::desc("my pass option help")); + +Here the command option is "``mypass``", with ``createDefaultMyPass`` as the +default creator. + +Using GDB with dynamically loaded passes +---------------------------------------- + +Unfortunately, using GDB with dynamically loaded passes is not as easy as it +should be. First of all, you can't set a breakpoint in a shared object that +has not been loaded yet, and second of all there are problems with inlined +functions in shared objects. Here are some suggestions to debugging your pass +with GDB. + +For sake of discussion, I'm going to assume that you are debugging a +transformation invoked by :program:`opt`, although nothing described here +depends on that. + +Setting a breakpoint in your pass +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +First thing you do is start gdb on the opt process: + +.. code-block:: console + + $ gdb opt + GNU gdb 5.0 + Copyright 2000 Free Software Foundation, Inc. + GDB is free software, covered by the GNU General Public License, and you are + welcome to change it and/or distribute copies of it under certain conditions. + Type "show copying" to see the conditions. + There is absolutely no warranty for GDB. Type "show warranty" for details. + This GDB was configured as "sparc-sun-solaris2.6"... + (gdb) + +Note that :program:`opt` has a lot of debugging information in it, so it takes +time to load. Be patient. Since we cannot set a breakpoint in our pass yet +(the shared object isn't loaded until runtime), we must execute the process, +and have it stop before it invokes our pass, but after it has loaded the shared +object. The most foolproof way of doing this is to set a breakpoint in +``PassManager::run`` and then run the process with the arguments you want: + +.. code-block:: console + + $ (gdb) break llvm::PassManager::run + Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70. + (gdb) run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption] + Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption] + Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70 + 70 bool PassManager::run(Module &M) { return PM->run(M); } + (gdb) + +Once the :program:`opt` stops in the ``PassManager::run`` method you are now +free to set breakpoints in your pass so that you can trace through execution or +do other standard debugging stuff. + +Miscellaneous Problems +^^^^^^^^^^^^^^^^^^^^^^ + +Once you have the basics down, there are a couple of problems that GDB has, +some with solutions, some without. + +* Inline functions have bogus stack information. In general, GDB does a pretty + good job getting stack traces and stepping through inline functions. When a + pass is dynamically loaded however, it somehow completely loses this + capability. The only solution I know of is to de-inline a function (move it + from the body of a class to a ``.cpp`` file). + +* Restarting the program breaks breakpoints. After following the information + above, you have succeeded in getting some breakpoints planted in your pass. + Nex thing you know, you restart the program (i.e., you type "``run``" again), + and you start getting errors about breakpoints being unsettable. The only + way I have found to "fix" this problem is to delete the breakpoints that are + already set in your pass, run the program, and re-set the breakpoints once + execution stops in ``PassManager::run``. + +Hopefully these tips will help with common case debugging situations. If you'd +like to contribute some tips of your own, just contact `Chris +<mailto:sabre@nondot.org>`_. + +Future extensions planned +------------------------- + +Although the LLVM Pass Infrastructure is very capable as it stands, and does +some nifty stuff, there are things we'd like to add in the future. Here is +where we are going: + +.. _writing-an-llvm-pass-SMP: + +Multithreaded LLVM +^^^^^^^^^^^^^^^^^^ + +Multiple CPU machines are becoming more common and compilation can never be +fast enough: obviously we should allow for a multithreaded compiler. Because +of the semantics defined for passes above (specifically they cannot maintain +state across invocations of their ``run*`` methods), a nice clean way to +implement a multithreaded compiler would be for the ``PassManager`` class to +create multiple instances of each pass object, and allow the separate instances +to be hacking on different parts of the program at the same time. + +This implementation would prevent each of the passes from having to implement +multithreaded constructs, requiring only the LLVM core to have locking in a few +places (for global resources). Although this is a simple extension, we simply +haven't had time (or multiprocessor machines, thus a reason) to implement this. +Despite that, we have kept the LLVM passes SMP ready, and you should too. + diff --git a/docs/subsystems.rst b/docs/subsystems.rst index 275955be6e..6c21d335af 100644 --- a/docs/subsystems.rst +++ b/docs/subsystems.rst @@ -23,14 +23,15 @@ Subsystem Documentation SourceLevelDebugging WritingAnLLVMBackend GarbageCollection + WritingAnLLVMPass .. FIXME: once LangRef is Sphinxified, HowToUseInstrMappings should be put under LangRef's toctree instead of this page's toctree. -* `Writing an LLVM Pass <WritingAnLLVMPass.html>`_ - +* :doc:`WritingAnLLVMPass` + Information on how to write LLVM transformations and analyses. - + * :doc:`WritingAnLLVMBackend` Information on how to write LLVM backends for machine targets. diff --git a/docs/userguides.rst b/docs/userguides.rst index cfb6dbeb5e..56eaf0886c 100644 --- a/docs/userguides.rst +++ b/docs/userguides.rst @@ -23,6 +23,7 @@ User Guides TestingGuide tutorial/index ReleaseNotes + Passes * :ref:`getting_started` @@ -58,10 +59,10 @@ User Guides A reference manual for the LLVM command line utilities ("man" pages for LLVM tools). -* `LLVM's Analysis and Transform Passes <Passes.html>`_ +* :doc:`Passes` A list of optimizations and analyses implemented in LLVM. - + * :ref:`faq` A list of common questions and problems and their solutions. |