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diff --git a/docs/main/WritingAnLLVMPass.html b/docs/main/WritingAnLLVMPass.html new file mode 100644 index 0000000000..f3e791f094 --- /dev/null +++ b/docs/main/WritingAnLLVMPass.html @@ -0,0 +1,1842 @@ +<!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="llvm.css" type="text/css"> +</head> +<body> + +<div class="doc_title"> + Writing an LLVM Pass +</div> + +<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="#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> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="introduction">Introduction - What is a pass?</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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="#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> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="quickstart">Quick Start - Writing hello world</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="makefile">Setting up the build environment</a> +</div> + +<div class="doc_text"> + + <p>First, 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>. Next, 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 + +# Tell the build system which LLVM libraries your pass needs. You'll probably +# need at least LLVMSystem.a, LLVMSupport.a, LLVMCore.a but possibly several +# others too. +LLVMLIBS = LLVMCore.a LLVMSupport.a LLVMSystem.a + +# 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 +<tt>Debug/lib/Hello.so</tt> shared object 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>Now that we have the build scripts set up, we just need to write the code for +the pass itself.</p> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="basiccode">Basic code required</a> +</div> + +<div class="doc_text"> + +<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 only visible 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> + +<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 a function at a +time.</p> + +<div class="doc_code"><pre> + static char ID; + Hello() : FunctionPass(&ID) {} +</pre></div><p> + +<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>" << F.getName() << "\n"; + <b>return false</b>; + } + }; <i>// end of struct Hello</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 pass so +initialization value is not important.</p> + +<div class="doc_code"><pre> + RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>", + false /* Only looks at CFG */, + false /* Analysis Pass */); +} <i>// end of anonymous namespace</i> +</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>". +Last two RegisterPass arguments are optional. Their default value is false. +If a pass walks CFG without modifying it then third argument is set to true. +If a pass is an analysis pass, for example dominator tree pass, then true +is supplied as 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>" << F.getName() << "\n"; + <b>return false</b>; + } + }; + + char Hello::ID = 0; + RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>"); +} +</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 +"<tt>Debug/lib/Hello.so</tt> file. 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> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="running">Running a pass with <tt>opt</tt></a> +</div> + +<div class="doc_text"> + +<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 the <tt>RegisterPass</tt> template, 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/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/lib/Hello.so -help +OVERVIEW: llvm .bc -> .bc modular optimizer + +USAGE: opt [options] <input bitcode> + +OPTIONS: + Optimizations available: +... + -funcresolve - Resolve Functions + -gcse - Global Common Subexpression Elimination + -globaldce - Dead Global Elimination + <b>-hello - Hello World Pass</b> + -indvars - Canonicalize Induction Variables + -inline - Function Integration/Inlining + -instcombine - Combine redundant instructions +... +</pre></div> + +<p>The pass name get 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/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 class="doc_section"> + <a name="passtype">Pass classes and requirements</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="ModulePass">The <tt>ModulePass</tt> class</a> +</div> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="runOnModule">The <tt>runOnModule</tt> method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_subsection"> + <a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a> +</div> + +<div class="doc_text"> + +<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 modify any <tt>Function</tt>s that are not in +the current SCC.</li> + +<li>... <em>not allowed</em> to inspect any Function's other than those in the +current SCC and the 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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doInitialization_scc">The <tt>doInitialization(CallGraph &)</tt> + method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="runOnSCC">The <tt>runOnSCC</tt> method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doFinalization_scc">The <tt>doFinalization(CallGraph + &)</tt> method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_subsection"> + <a name="FunctionPass">The <tt>FunctionPass</tt> class</a> +</div> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doInitialization_mod">The <tt>doInitialization(Module &)</tt> + method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="runOnFunction">The <tt>runOnFunction</tt> method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doFinalization_mod">The <tt>doFinalization(Module + &)</tt> method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_subsection"> + <a name="LoopPass">The <tt>LoopPass</tt> class </a> +</div> + +<div class="doc_text"> + +<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> +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doInitialization_loop">The <tt>doInitialization(Loop *, + LPPassManager &)</tt> + method</a> +</div> + +<div class="doc_text"> + +<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> + + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="runOnLoop">The <tt>runOnLoop</tt> method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_subsection"> + <a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a> +</div> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doInitialization_fn">The <tt>doInitialization(Function + &)</tt> method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="doFinalization_fn">The <tt>doFinalization(Function &)</tt> + method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_subsection"> + <a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a> +</div> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction + &MF)</tt> method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_section"> + <a name="registration">Pass registration</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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, which requires you to pass at least two +parameters. The first 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 second 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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="print">The <tt>print</tt> method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_section"> + <a name="interaction">Specifying interactions between passes</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a> +</div> + +<div class="doc_text"> +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a> +</div> + +<div class="doc_text"> +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a> +</div> + +<div class="doc_text"> + +<div class="doc_code"><pre> + <i>// This is an example implementation from an analysis, which does not modify + // the program at all, yet has a prerequisite.</i> + <b>void</b> <a href="http://llvm.org/doxygen/classllvm_1_1PostDominanceFrontier.html">PostDominanceFrontier</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> { + AU.setPreservesAll(); + AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1PostDominatorTree.html">PostDominatorTree</a>>(); + } +</pre></div> + +<p>and:</p> + +<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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="getAnalysis">The <tt>getAnalysis<></tt> and +<tt>getAnalysisIfAvailable<></tt> methods</a> +</div> + +<div class="doc_text"> + +<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 class="doc_section"> + <a name="analysisgroup">Implementing Analysis Groups</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="agconcepts">Analysis Group Concepts</a> +</div> + +<div class="doc_text"> + +<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>RegisterAnalysisGroup</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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a> +</div> + +<div class="doc_text"> + +<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis +group itself as well as add pass implementations to the analysis group. First, +an analysis 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> Analysis Group implementations <b>must</b> be registered normally...</i> + RegisterPass<FancyAA> + B("<i>somefancyaa</i>", "<i>A more complex alias analysis implementation</i>"); + + //<i> Declare that we implement the AliasAnalysis interface</i> + RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> C(B); +} +</pre></div> + +<p>This just shows a class <tt>FancyAA</tt> that is registered normally, then +uses the <tt>RegisterAnalysisGroup</tt> template 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 template. A single pass may join multiple different analysis groups with +no problem.</p> + +<div class="doc_code"><pre> +<b>namespace</b> { + //<i> Analysis Group implementations <b>must</b> be registered normally...</i> + RegisterPass<<a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a>> + D("<i>basicaa</i>", "<i>Basic Alias Analysis (default AA impl)</i>"); + + //<i> Declare that we implement the AliasAnalysis interface</i> + RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, <b>true</b>> E(D); +} +</pre></div> + +<p>Here we show how the default implementation is specified (using the extra +argument to the <tt>RegisterAnalysisGroup</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 class="doc_section"> + <a name="passStatistics">Pass Statistics</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> +<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> + + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="passmanager">What PassManager does</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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 consequtive <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/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/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/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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="releaseMemory">The <tt>releaseMemory</tt> method</a> +</div> + +<div class="doc_text"> + +<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 class="doc_section"> + <a name="registering">Registering dynamically loaded passes</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsection"> + <a name="registering_existing">Using existing registries</a> +</div> + +<div class="doc_text"> + +<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 +.cpp 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> + + +<!-- _______________________________________________________________________ --> +<div class="doc_subsection"> + <a name="registering_new">Creating new registries</a> +</div> + +<div class="doc_text"> + +<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 class="doc_section"> + <a name="debughints">Using GDB with dynamically loaded passes</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="breakpoint">Setting a breakpoint in your pass</a> +</div> + +<div class="doc_text"> + +<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/lib/[libname].so -[passoption]</b> +Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug/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> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="debugmisc">Miscellaneous Problems</a> +</div> + +<div class="doc_text"> + +<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 class="doc_section"> + <a name="future">Future extensions planned</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</div> + +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="SMP">Multithreaded LLVM</a> +</div> + +<div class="doc_text"> + +<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> + +<!-- *********************************************************************** --> +<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> |