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| author | Michael J. Spencer <bigcheesegs@gmail.com> | 2012-06-18 20:21:38 +0000 |
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| committer | Michael J. Spencer <bigcheesegs@gmail.com> | 2012-06-18 20:21:38 +0000 |
| commit | 0ed5cf4fc1f6d7947687114c9d0cbe0d1ba1d883 (patch) | |
| tree | 3caa77cec8f49d08ce7fdfd9b36017c888ecb361 | |
| parent | 9ac0f1d7255d7941c331b0a62a830a3b033358fd (diff) | |
| download | llvm-0ed5cf4fc1f6d7947687114c9d0cbe0d1ba1d883.tar.gz llvm-0ed5cf4fc1f6d7947687114c9d0cbe0d1ba1d883.tar.bz2 llvm-0ed5cf4fc1f6d7947687114c9d0cbe0d1ba1d883.tar.xz | |
[docs] Port FAQ over to Sphinx.
Patch by Mikael Lyngvig!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158677 91177308-0d34-0410-b5e6-96231b3b80d8
| -rw-r--r-- | docs/FAQ.html | 758 | ||||
| -rw-r--r-- | docs/FAQ.rst | 464 | ||||
| -rw-r--r-- | docs/userguides.rst | 3 |
3 files changed, 466 insertions, 759 deletions
diff --git a/docs/FAQ.html b/docs/FAQ.html deleted file mode 100644 index 60192a99ee..0000000000 --- a/docs/FAQ.html +++ /dev/null @@ -1,758 +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>LLVM: Frequently Asked Questions</title> - <style type="text/css"> - @import url("_static/llvm.css"); - .question { font-weight: bold } - .answer { margin-left: 2em } - </style> -</head> -<body> - -<h1> - LLVM: Frequently Asked Questions -</h1> - -<ol> - <li><a href="#license">License</a> - <ol> - <li>Does the University of Illinois Open Source License really qualify as an - "open source" license?</li> - - <li>Can I modify LLVM source code and redistribute the modified source?</li> - - <li>Can I modify LLVM source code and redistribute binaries or other tools - based on it, without redistributing the source?</li> - </ol></li> - - <li><a href="#source">Source code</a> - <ol> - <li>In what language is LLVM written?</li> - - <li>How portable is the LLVM source code?</li> - </ol></li> - - <li><a href="#build">Build Problems</a> - <ol> - <li>When I run configure, it finds the wrong C compiler.</li> - - <li>The <tt>configure</tt> script finds the right C compiler, but it uses - the LLVM linker from a previous build. What do I do?</li> - - <li>When creating a dynamic library, I get a strange GLIBC error.</li> - - <li>I've updated my source tree from Subversion, and now my build is trying - to use a file/directory that doesn't exist.</li> - - <li>I've modified a Makefile in my source tree, but my build tree keeps - using the old version. What do I do?</li> - - <li>I've upgraded to a new version of LLVM, and I get strange build - errors.</li> - - <li>I've built LLVM and am testing it, but the tests freeze.</li> - - <li>Why do test results differ when I perform different types of - builds?</li> - - <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li> - - <li>Compiling LLVM with GCC succeeds, but the resulting tools do not work, - what can be wrong?</li> - - <li>After Subversion update, rebuilding gives the error "No rule to make - target".</li> - - </ol></li> - - <li><a href="#felangs">Source Languages</a> - <ol> - <li><a href="#langs">What source languages are supported?</a></li> - - <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How - should I interface with the LLVM middle-end optimizers and back-end code - generators?</a></li> - - <li><a href="#langhlsupp">What support is there for higher level source - language constructs for building a compiler?</a></li> - - <li><a href="GetElementPtr.html">I don't understand the GetElementPtr - instruction. Help!</a></li> - </ol> - - <li><a href="#cfe">Using the C and C++ Front Ends</a> - <ol> - <li><a href="#platformindependent">Can I compile C or C++ code to - platform-independent LLVM bitcode?</a></li> - </ol> - </li> - - <li><a href="#cfe_code">Questions about code generated by the demo page</a> - <ol> - <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and - <tt>_GLOBAL__I_a...</tt> stuff that happens when I - #include <iostream>?</a></li> - - <li><a href="#codedce">Where did all of my code go??</a></li> - - <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in - my code?</a></li> - - <li><a href="#callconvwrong">Why does instcombine + simplifycfg turn - a call to a function with a mismatched calling convention into "unreachable"? - Why not make the verifier reject it?</a></li> - </ol> - </li> -</ol> - -<div class="doc_author"> - <p>Written by <a href="http://llvm.org/">The LLVM Team</a></p> -</div> - - -<!-- *********************************************************************** --> -<h2> - <a name="license">License</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<div class="question"> -<p>Does the University of Illinois Open Source License really qualify as an - "open source" license?</p> -</div> - -<div class="answer"> -<p>Yes, the license - is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by - the Open Source Initiative (OSI).</p> -</div> - -<div class="question"> -<p>Can I modify LLVM source code and redistribute the modified source?</p> -</div> - -<div class="answer"> -<p>Yes. The modified source distribution must retain the copyright notice and - follow the three bulletted conditions listed in - the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM - license</a>.</p> -</div> - -<div class="question"> -<p>Can I modify LLVM source code and redistribute binaries or other tools based - on it, without redistributing the source?</p> -</div> - -<div class="answer"> -<p>Yes. This is why we distribute LLVM under a less restrictive license than - GPL, as explained in the first question above.</p> -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="source">Source Code</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<div class="question"> -<p>In what language is LLVM written?</p> -</div> - -<div class="answer"> -<p>All of the LLVM tools and libraries are written in C++ with extensive use of - the STL.</p> -</div> - -<div class="question"> -<p>How portable is the LLVM source code?</p> -</div> - -<div class="answer"> -<p>The LLVM source code should be portable to most modern UNIX-like operating -systems. Most of the code is written in standard C++ with operating system -services abstracted to a support library. The tools required to build and test -LLVM have been ported to a plethora of platforms.</p> - -<p>Some porting problems may exist in the following areas:</p> - -<ul> - <li>The autoconf/makefile build system relies heavily on UNIX shell tools, - like the Bourne Shell and sed. Porting to systems without these tools - (MacOS 9, Plan 9) Will require more effort.</li> -</ul> - -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="build">Build Problems</a> -</h2> -<!-- *********************************************************************** --> - -<div> - -<div class="question"> -<p>When I run configure, it finds the wrong C compiler.</p> -</div> - -<div class="answer"> -<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then - <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> - and <tt>CXX</tt> for the C and C++ compiler, respectively.</p> - -<p>If <tt>configure</tt> finds the wrong compiler, either adjust your - <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt> - explicitly.</p> - -</div> - -<div class="question"> -<p>The <tt>configure</tt> script finds the right C compiler, but it uses the - LLVM tools from a previous build. What do I do?</p> -</div> - -<div class="answer"> -<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so - if it's grabbing the wrong linker/assembler/etc, there are two ways to fix - it:</p> - -<ol> - <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct - program appears first in the <tt>PATH</tt>. This may work, but may not be - convenient when you want them <i>first</i> in your path for other - work.</p></li> - - <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is - correct. In a Bourne compatible shell, the syntax would be:</p> - -<pre class="doc_code"> -% PATH=[the path without the bad program] ./configure ... -</pre> - - <p>This is still somewhat inconvenient, but it allows <tt>configure</tt> - to do its work without having to adjust your <tt>PATH</tt> - permanently.</p></li> -</ol> -</div> - -<div class="question"> -<p>When creating a dynamic library, I get a strange GLIBC error.</p> -</div> - -<div class="answer"> -<p>Under some operating systems (i.e. Linux), libtool does not work correctly if - GCC was compiled with the --disable-shared option. To work around this, - install your own version of GCC that has shared libraries enabled by - default.</p> -</div> - -<div class="question"> -<p>I've updated my source tree from Subversion, and now my build is trying to - use a file/directory that doesn't exist.</p> -</div> - -<div class="answer"> -<p>You need to re-run configure in your object directory. When new Makefiles - are added to the source tree, they have to be copied over to the object tree - in order to be used by the build.</p> -</div> - -<div class="question"> -<p>I've modified a Makefile in my source tree, but my build tree keeps using the - old version. What do I do?</p> -</div> - -<div class="answer"> -<p>If the Makefile already exists in your object tree, you can just run the - following command in the top level directory of your object tree:</p> - -<pre class="doc_code"> -% ./config.status <relative path to Makefile> -</pre> - -<p>If the Makefile is new, you will have to modify the configure script to copy - it over.</p> -</div> - -<div class="question"> -<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p> -</div> - -<div class="answer"> - -<p>Sometimes, changes to the LLVM source code alters how the build system works. - Changes in libtool, autoconf, or header file dependencies are especially - prone to this sort of problem.</p> - -<p>The best thing to try is to remove the old files and re-build. In most - cases, this takes care of the problem. To do this, just type <tt>make - clean</tt> and then <tt>make</tt> in the directory that fails to build.</p> -</div> - -<div class="question"> -<p>I've built LLVM and am testing it, but the tests freeze.</p> -</div> - -<div class="answer"> -<p>This is most likely occurring because you built a profile or release - (optimized) build of LLVM and have not specified the same information on the - <tt>gmake</tt> command line.</p> - -<p>For example, if you built LLVM with the command:</p> - -<pre class="doc_code"> -% gmake ENABLE_PROFILING=1 -</pre> - -<p>...then you must run the tests with the following commands:</p> - -<pre class="doc_code"> -% cd llvm/test -% gmake ENABLE_PROFILING=1 -</pre> -</div> - -<div class="question"> -<p>Why do test results differ when I perform different types of builds?</p> -</div> - -<div class="answer"> -<p>The LLVM test suite is dependent upon several features of the LLVM tools and - libraries.</p> - -<p>First, the debugging assertions in code are not enabled in optimized or - profiling builds. Hence, tests that used to fail may pass.</p> - -<p>Second, some tests may rely upon debugging options or behavior that is only - available in the debug build. These tests will fail in an optimized or - profile build.</p> -</div> - -<div class="question"> -<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p> -</div> - -<div class="answer"> -<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in - GCC</a>, and affects projects other than LLVM. Try upgrading or downgrading - your GCC.</p> -</div> - -<div class="question"> -<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what - can be wrong?</p> -</div> - -<div class="answer"> -<p>Several versions of GCC have shown a weakness in miscompiling the LLVM - codebase. Please consult your compiler version (<tt>gcc --version</tt>) to - find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>. - If so, your only option is to upgrade GCC to a known good version.</p> -</div> - -<div class="question"> -<p>After Subversion update, rebuilding gives the error "No rule to make - target".</p> -</div> - -<div class="answer"> -<p>If the error is of the form:</p> - -<pre class="doc_code"> -gmake[2]: *** No rule to make target `/path/to/somefile', needed by -`/path/to/another/file.d'.<br> -Stop. -</pre> - -<p>This may occur anytime files are moved within the Subversion repository or - removed entirely. In this case, the best solution is to erase all - <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p> - -<pre class="doc_code"> -% cd $LLVM_OBJ_DIR -% rm -f `find . -name \*\.d` -% gmake -</pre> - -<p>In other cases, it may be necessary to run <tt>make clean</tt> before - rebuilding.</p> -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="felangs">Source Languages</a> -</h2> - -<div> - -<div class="question"> -<p><a name="langs">What source languages are supported?</a></p> -</div> - -<div class="answer"> -<p>LLVM currently has full support for C and C++ source languages. These are - available through both <a href="http://clang.llvm.org/">Clang</a> and - <a href="http://dragonegg.llvm.org/">DragonEgg</a>.</p> - -<p>The PyPy developers are working on integrating LLVM into the PyPy backend so - that PyPy language can translate to LLVM.</p> -</div> - -<div class="question"> -<p><a name="langirgen">I'd like to write a self-hosting LLVM compiler. How - should I interface with the LLVM middle-end optimizers and back-end code - generators?</a></p> -</div> - -<div class="answer"> -<p>Your compiler front-end will communicate with LLVM by creating a module in - the LLVM intermediate representation (IR) format. Assuming you want to write - your language's compiler in the language itself (rather than C++), there are - 3 major ways to tackle generating LLVM IR from a front-end:</p> - -<ul> - <li><strong>Call into the LLVM libraries code using your language's FFI - (foreign function interface).</strong> - - <ul> - <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc - format</li> - - <li><em>for:</em> enables running LLVM optimization passes without a - emit/parse overhead</li> - - <li><em>for:</em> adapts well to a JIT context</li> - - <li><em>against:</em> lots of ugly glue code to write</li> - </ul></li> - - <li> <strong>Emit LLVM assembly from your compiler's native language.</strong> - <ul> - <li><em>for:</em> very straightforward to get started</li> - - <li><em>against:</em> the .ll parser is slower than the bitcode reader - when interfacing to the middle end</li> - - <li><em>against:</em> you'll have to re-engineer the LLVM IR object model - and asm writer in your language</li> - - <li><em>against:</em> it may be harder to track changes to the IR</li> - </ul></li> - - <li><strong>Emit LLVM bitcode from your compiler's native language.</strong> - - <ul> - <li><em>for:</em> can use the more-efficient bitcode reader when - interfacing to the middle end</li> - - <li><em>against:</em> you'll have to re-engineer the LLVM IR object - model and bitcode writer in your language</li> - - <li><em>against:</em> it may be harder to track changes to the IR</li> - </ul></li> -</ul> - -<p>If you go with the first option, the C bindings in include/llvm-c should help - a lot, since most languages have strong support for interfacing with C. The - most common hurdle with calling C from managed code is interfacing with the - garbage collector. The C interface was designed to require very little memory - management, and so is straightforward in this regard.</p> -</div> - -<div class="question"> -<p><a name="langhlsupp">What support is there for a higher level source language - constructs for building a compiler?</a></p> -</div> - -<div class="answer"> -<p>Currently, there isn't much. LLVM supports an intermediate representation - which is useful for code representation but will not support the high level - (abstract syntax tree) representation needed by most compilers. There are no - facilities for lexical nor semantic analysis.</p> -</div> - -<div class="question"> -<p><a name="getelementptr">I don't understand the GetElementPtr - instruction. Help!</a></p> -</div> - -<div class="answer"> -<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP - Instruction</a>.</p> -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="cfe">Using the C and C++ Front Ends</a> -</h2> - -<div> - -<div class="question"> -<p><a name="platformindependent">Can I compile C or C++ code to - platform-independent LLVM bitcode?</a></p> -</div> - -<div class="answer"> -<p>No. C and C++ are inherently platform-dependent languages. The most obvious - example of this is the preprocessor. A very common way that C code is made - portable is by using the preprocessor to include platform-specific code. In - practice, information about other platforms is lost after preprocessing, so - the result is inherently dependent on the platform that the preprocessing was - targeting.</p> - -<p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to - vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to - a constant immediately, thus hard-wiring a platform-specific detail.</p> - -<p>Also, since many platforms define their ABIs in terms of C, and since LLVM is - lower-level than C, front-ends currently must emit platform-specific IR in - order to have the result conform to the platform ABI.</p> -</div> - -</div> - -<!-- *********************************************************************** --> -<h2> - <a name="cfe_code">Questions about code generated by the demo page</a> -</h2> - -<div> - -<div class="question"> -<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and - <tt>_GLOBAL__I_a...</tt> stuff that happens when I <tt>#include - <iostream></tt>?</a></p> -</div> - -<div class="answer"> -<p>If you <tt>#include</tt> the <tt><iostream></tt> header into a C++ - translation unit, the file will probably use - the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects. However, C++ - does not guarantee an order of initialization between static objects in - different translation units, so if a static ctor/dtor in your .cpp file - used <tt>std::cout</tt>, for example, the object would not necessarily be - automatically initialized before your use.</p> - -<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the - STL that we use declares a static object that gets created in every - translation unit that includes <tt><iostream></tt>. This object has a - static constructor and destructor that initializes and destroys the global - iostream objects before they could possibly be used in the file. The code - that you see in the .ll file corresponds to the constructor and destructor - registration code. -</p> - -<p>If you would like to make it easier to <b>understand</b> the LLVM code - generated by the compiler in the demo page, consider using <tt>printf()</tt> - instead of <tt>iostream</tt>s to print values.</p> -</div> - -<!--=========================================================================--> - -<div class="question"> -<p><a name="codedce">Where did all of my code go??</a></p> -</div> - -<div class="answer"> -<p>If you are using the LLVM demo page, you may often wonder what happened to - all of the code that you typed in. Remember that the demo script is running - the code through the LLVM optimizers, so if your code doesn't actually do - anything useful, it might all be deleted.</p> - -<p>To prevent this, make sure that the code is actually needed. For example, if - you are computing some expression, return the value from the function instead - of leaving it in a local variable. If you really want to constrain the - optimizer, you can read from and assign to <tt>volatile</tt> global - variables.</p> -</div> - -<!--=========================================================================--> - -<div class="question"> -<p><a name="undef">What is this "<tt>undef</tt>" thing that shows up in my - code?</a></p> -</div> - -<div class="answer"> -<p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of - representing a value that is not defined. You can get these if you do not - initialize a variable before you use it. For example, the C function:</p> - -<pre class="doc_code"> -int X() { int i; return i; } -</pre> - -<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a - value specified for it.</p> -</div> - -<!--=========================================================================--> - -<div class="question"> -<p><a name="callconvwrong">Why does instcombine + simplifycfg turn - a call to a function with a mismatched calling convention into "unreachable"? - Why not make the verifier reject it?</a></p> -</div> - -<div class="answer"> -<p>This is a common problem run into by authors of front-ends that are using -custom calling conventions: you need to make sure to set the right calling -convention on both the function and on each call to the function. For example, -this code:</p> - -<pre class="doc_code"> -define fastcc void @foo() { - ret void -} -define void @bar() { - call void @foo() - ret void -} -</pre> - -<p>Is optimized to:</p> - -<pre class="doc_code"> -define fastcc void @foo() { - ret void -} -define void @bar() { - unreachable -} -</pre> - -<p>... with "opt -instcombine -simplifycfg". This often bites people because -"all their code disappears". Setting the calling convention on the caller and -callee is required for indirect calls to work, so people often ask why not make -the verifier reject this sort of thing.</p> - -<p>The answer is that this code has undefined behavior, but it is not illegal. -If we made it illegal, then every transformation that could potentially create -this would have to ensure that it doesn't, and there is valid code that can -create this sort of construct (in dead code). The sorts of things that can -cause this to happen are fairly contrived, but we still need to accept them. -Here's an example:</p> - -<pre class="doc_code"> -define fastcc void @foo() { - ret void -} -define internal void @bar(void()* %FP, i1 %cond) { - br i1 %cond, label %T, label %F -T: - call void %FP() - ret void -F: - call fastcc void %FP() - ret void -} -define void @test() { - %X = or i1 false, false - call void @bar(void()* @foo, i1 %X) - ret void -} -</pre> - -<p>In this example, "test" always passes @foo/false into bar, which ensures that - it is dynamically called with the right calling conv (thus, the code is - perfectly well defined). If you run this through the inliner, you get this - (the explicit "or" is there so that the inliner doesn't dead code eliminate - a bunch of stuff): -</p> - -<pre class="doc_code"> -define fastcc void @foo() { - ret void -} -define void @test() { - %X = or i1 false, false - br i1 %X, label %T.i, label %F.i -T.i: - call void @foo() - br label %bar.exit -F.i: - call fastcc void @foo() - br label %bar.exit -bar.exit: - ret void -} -</pre> - -<p>Here you can see that the inlining pass made an undefined call to @foo with - the wrong calling convention. We really don't want to make the inliner have - to know about this sort of thing, so it needs to be valid code. In this case, - dead code elimination can trivially remove the undefined code. However, if %X - was an input argument to @test, the inliner would produce this: -</p> - -<pre class="doc_code"> -define fastcc void @foo() { - ret void -} - -define void @test(i1 %X) { - br i1 %X, label %T.i, label %F.i -T.i: - call void @foo() - br label %bar.exit -F.i: - call fastcc void @foo() - br label %bar.exit -bar.exit: - ret void -} -</pre> - -<p>The interesting thing about this is that %X <em>must</em> be false for the -code to be well-defined, but no amount of dead code elimination will be able to -delete the broken call as unreachable. However, since instcombine/simplifycfg -turns the undefined call into unreachable, we end up with a branch on a -condition that goes to unreachable: a branch to unreachable can never happen, so -"-inline -instcombine -simplifycfg" is able to produce:</p> - -<pre class="doc_code"> -define fastcc void @foo() { - ret void -} -define void @test(i1 %X) { -F.i: - call fastcc void @foo() - ret void -} -</pre> - -</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="http://llvm.org/">LLVM Compiler Infrastructure</a><br> - Last modified: $Date$ -</address> - -</body> -</html> diff --git a/docs/FAQ.rst b/docs/FAQ.rst new file mode 100644 index 0000000000..b0e3ca0456 --- /dev/null +++ b/docs/FAQ.rst @@ -0,0 +1,464 @@ +.. _faq: + +================================ +Frequently Asked Questions (FAQ) +================================ + +.. contents:: + :local: + + +License +======= + +Does the University of Illinois Open Source License really qualify as an "open source" license? +----------------------------------------------------------------------------------------------- +Yes, the license is `certified +<http://www.opensource.org/licenses/UoI-NCSA.php>`_ by the Open Source +Initiative (OSI). + + +Can I modify LLVM source code and redistribute the modified source? +------------------------------------------------------------------- +Yes. The modified source distribution must retain the copyright notice and +follow the three bulletted conditions listed in the `LLVM license +<http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT>`_. + + +Can I modify the LLVM source code and redistribute binaries or other tools based on it, without redistributing the source? +-------------------------------------------------------------------------------------------------------------------------- +Yes. This is why we distribute LLVM under a less restrictive license than GPL, +as explained in the first question above. + + +Source Code +=========== + +In what language is LLVM written? +--------------------------------- +All of the LLVM tools and libraries are written in C++ with extensive use of +the STL. + + +How portable is the LLVM source code? +------------------------------------- +The LLVM source code should be portable to most modern Unix-like operating +systems. Most of the code is written in standard C++ with operating system +services abstracted to a support library. The tools required to build and +test LLVM have been ported to a plethora of platforms. + +Some porting problems may exist in the following areas: + +* The autoconf/makefile build system relies heavily on UNIX shell tools, + like the Bourne Shell and sed. Porting to systems without these tools + (MacOS 9, Plan 9) will require more effort. + + +Build Problems +============== + +When I run configure, it finds the wrong C compiler. +---------------------------------------------------- +The ``configure`` script attempts to locate first ``gcc`` and then ``cc``, +unless it finds compiler paths set in ``CC`` and ``CXX`` for the C and C++ +compiler, respectively. + +If ``configure`` finds the wrong compiler, either adjust your ``PATH`` +environment variable or set ``CC`` and ``CXX`` explicitly. + + +The ``configure`` script finds the right C compiler, but it uses the LLVM tools from a previous build. What do I do? +--------------------------------------------------------------------------------------------------------------------- +The ``configure`` script uses the ``PATH`` to find executables, so if it's +grabbing the wrong linker/assembler/etc, there are two ways to fix it: + +#. Adjust your ``PATH`` environment variable so that the correct program + appears first in the ``PATH``. This may work, but may not be convenient + when you want them *first* in your path for other work. + +#. Run ``configure`` with an alternative ``PATH`` that is correct. In a + Bourne compatible shell, the syntax would be: + +.. code-block:: bash + + % PATH=[the path without the bad program] ./configure ... + +This is still somewhat inconvenient, but it allows ``configure`` to do its +work without having to adjust your ``PATH`` permanently. + + +When creating a dynamic library, I get a strange GLIBC error. +------------------------------------------------------------- +Under some operating systems (i.e. Linux), libtool does not work correctly if +GCC was compiled with the ``--disable-shared option``. To work around this, +install your own version of GCC that has shared libraries enabled by default. + + +I've updated my source tree from Subversion, and now my build is trying to use a file/directory that doesn't exist. +------------------------------------------------------------------------------------------------------------------- +You need to re-run configure in your object directory. When new Makefiles +are added to the source tree, they have to be copied over to the object tree +in order to be used by the build. + + +I've modified a Makefile in my source tree, but my build tree keeps using the old version. What do I do? +--------------------------------------------------------------------------------------------------------- +If the Makefile already exists in your object tree, you can just run the +following command in the top level directory of your object tree: + +.. code-block:: bash + + % ./config.status <relative path to Makefile>; + +If the Makefile is new, you will have to modify the configure script to copy +it over. + + +I've upgraded to a new version of LLVM, and I get strange build errors. +----------------------------------------------------------------------- +Sometimes, changes to the LLVM source code alters how the build system works. +Changes in ``libtool``, ``autoconf``, or header file dependencies are +especially prone to this sort of problem. + +The best thing to try is to remove the old files and re-build. In most cases, +this takes care of the problem. To do this, just type ``make clean`` and then +``make`` in the directory that fails to build. + + +I've built LLVM and am testing it, but the tests freeze. +-------------------------------------------------------- +This is most likely occurring because you built a profile or release +(optimized) build of LLVM and have not specified the same information on the +``gmake`` command line. + +For example, if you built LLVM with the command: + +.. code-block:: bash + + % gmake ENABLE_PROFILING=1 + +...then you must run the tests with the following commands: + +.. code-block:: bash + + % cd llvm/test + % gmake ENABLE_PROFILING=1 + +Why do test results differ when I perform different types of builds? +-------------------------------------------------------------------- +The LLVM test suite is dependent upon several features of the LLVM tools and +libraries. + +First, the debugging assertions in code are not enabled in optimized or +profiling builds. Hence, tests that used to fail may pass. + +Second, some tests may rely upon debugging options or behavior that is only +available in the debug build. These tests will fail in an optimized or +profile build. + + +Compiling LLVM with GCC 3.3.2 fails, what should I do? +------------------------------------------------------ +This is `a bug in GCC <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392>`_, +and affects projects other than LLVM. Try upgrading or downgrading your GCC. + + +Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong? +----------------------------------------------------------------------------------------- +Several versions of GCC have shown a weakness in miscompiling the LLVM +codebase. Please consult your compiler version (``gcc --version``) to find +out whether it is `broken <GettingStarted.html#brokengcc>`_. If so, your only +option is to upgrade GCC to a known good version. + + +After Subversion update, rebuilding gives the error "No rule to make target". +----------------------------------------------------------------------------- +If the error is of the form: + +.. code-block:: bash + + gmake[2]: *** No rule to make target `/path/to/somefile', + needed by `/path/to/another/file.d'. + Stop. + +This may occur anytime files are moved within the Subversion repository or +removed entirely. In this case, the best solution is to erase all ``.d`` +files, which list dependencies for source files, and rebuild: + +.. code-block:: bash + + % cd $LLVM_OBJ_DIR + % rm -f `find . -name \*\.d` + % gmake + +In other cases, it may be necessary to run ``make clean`` before rebuilding. + + +Source Languages +================ + +What source languages are supported? +------------------------------------ +LLVM currently has full support for C and C++ source languages. These are +available through both `Clang <http://clang.llvm.org/>`_ and `DragonEgg +<http://dragonegg.llvm.org/>`_. + +The PyPy developers are working on integrating LLVM into the PyPy backend so +that PyPy language can translate to LLVM. + + +I'd like to write a self-hosting LLVM compiler. How should I interface with the LLVM middle-end optimizers and back-end code generators? +---------------------------------------------------------------------------------------------------------------------------------------- +Your compiler front-end will communicate with LLVM by creating a module in the +LLVM intermediate representation (IR) format. Assuming you want to write your +language's compiler in the language itself (rather than C++), there are 3 +major ways to tackle generating LLVM IR from a front-end: + +1. **Call into the LLVM libraries code using your language's FFI (foreign + function interface).** + + * *for:* best tracks changes to the LLVM IR, .ll syntax, and .bc format + + * *for:* enables running LLVM optimization passes without a emit/parse + overhead + + * *for:* adapts well to a JIT context + + * *against:* lots of ugly glue code to write + +2. **Emit LLVM assembly from your compiler's native language.** + + * *for:* very straightforward to get started + + * *against:* the .ll parser is slower than the bitcode reader when + interfacing to the middle end + + * *against:* it may be harder to track changes to the IR + +3. **Emit LLVM bitcode from your compiler's native language.** + + * *for:* can use the more-efficient bitcode reader when interfacing to the + middle end + + * *against:* you'll have to re-engineer the LLVM IR object model and bitcode + writer in your language + + * *against:* it may be harder to track changes to the IR + +If you go with the first option, the C bindings in include/llvm-c should help +a lot, since most languages have strong support for interfacing with C. The +most common hurdle with calling C from managed code is interfacing with the +garbage collector. The C interface was designed to require very little memory +management, and so is straightforward in this regard. + +What support is there for a higher level source language constructs for building a compiler? +-------------------------------------------------------------------------------------------- +Currently, there isn't much. LLVM supports an intermediate representation +which is useful for code representation but will not support the high level +(abstract syntax tree) representation needed by most compilers. There are no +facilities for lexical nor semantic analysis. + + +I don't understand the ``GetElementPtr`` instruction. Help! +----------------------------------------------------------- +See `The Often Misunderstood GEP Instruction <GetElementPtr.html>`_. + + +Using the C and C++ Front Ends +============================== + +Can I compile C or C++ code to platform-independent LLVM bitcode? +----------------------------------------------------------------- +No. C and C++ are inherently platform-dependent languages. The most obvious +example of this is the preprocessor. A very common way that C code is made +portable is by using the preprocessor to include platform-specific code. In +practice, information about other platforms is lost after preprocessing, so +the result is inherently dependent on the platform that the preprocessing was +targeting. + +Another example is ``sizeof``. It's common for ``sizeof(long)`` to vary +between platforms. In most C front-ends, ``sizeof`` is expanded to a +constant immediately, thus hard-wiring a platform-specific detail. + +Also, since many platforms define their ABIs in terms of C, and since LLVM is +lower-level than C, front-ends currently must emit platform-specific IR in +order to have the result conform to the platform ABI. + + +Questions about code generated by the demo page +=============================================== + +What is this ``llvm.global_ctors`` and ``_GLOBAL__I_a...`` stuff that happens when I ``#include <iostream>``? +------------------------------------------------------------------------------------------------------------- +If you ``#include`` the ``<iostream>`` header into a C++ translation unit, +the file will probably use the ``std::cin``/``std::cout``/... global objects. +However, C++ does not guarantee an order of initialization between static +objects in different translation units, so if a static ctor/dtor in your .cpp +file used ``std::cout``, for example, the object would not necessarily be +automatically initialized before your use. + +To make ``std::cout`` and friends work correctly in these scenarios, the STL +that we use declares a static object that gets created in every translation +unit that includes ``<iostream>``. This object has a static constructor +and destructor that initializes and destroys the global iostream objects +before they could possibly be used in the file. The code that you see in the +``.ll`` file corresponds to the constructor and destructor registration code. + +If you would like to make it easier to *understand* the LLVM code generated +by the compiler in the demo page, consider using ``printf()`` instead of +``iostream``\s to print values. + + +Where did all of my code go?? +----------------------------- +If you are using the LLVM demo page, you may often wonder what happened to +all of the code that you typed in. Remember that the demo script is running +the code through the LLVM optimizers, so if your code doesn't actually do +anything useful, it might all be deleted. + +To prevent this, make sure that the code is actually needed. For example, if +you are computing some expression, return the value from the function instead +of leaving it in a local variable. If you really want to constrain the +optimizer, you can read from and assign to ``volatile`` global variables. + + +What is this "``undef``" thing that shows up in my code? +-------------------------------------------------------- +``undef`` is the LLVM way of representing a value that is not defined. You +can get these if you do not initialize a variable before you use it. For +example, the C function: + +.. code-block:: c + + int X() { int i; return i; } + +Is compiled to "``ret i32 undef``" because "``i``" never has a value specified +for it. + + +Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into "unreachable"? Why not make the verifier reject it? +---------------------------------------------------------------------------------------------------------------------------------------------------------- +This is a common problem run into by authors of front-ends that are using +custom calling conventions: you need to make sure to set the right calling +convention on both the function and on each call to the function. For +example, this code: + +.. code-block:: llvm + + define fastcc void @foo() { + ret void + } + define void @bar() { + call void @foo() + ret void + } + +Is optimized to: + +.. code-block:: llvm + + define fastcc void @foo() { + ret void + } + define void @bar() { + unreachable + } + +... with "``opt -instcombine -simplifycfg``". This often bites people because +"all their code disappears". Setting the calling convention on the caller and +callee is required for indirect calls to work, so people often ask why not +make the verifier reject this sort of thing. + +The answer is that this code has undefined behavior, but it is not illegal. +If we made it illegal, then every transformation that could potentially create +this would have to ensure that it doesn't, and there is valid code that can +create this sort of construct (in dead code). The sorts of things that can +cause this to happen are fairly contrived, but we still need to accept them. +Here's an example: + +.. code-block:: llvm + + define fastcc void @foo() { + ret void + } + define internal void @bar(void()* %FP, i1 %cond) { + br i1 %cond, label %T, label %F + T: + call void %FP() + ret void + F: + call fastcc void %FP() + ret void + } + define void @test() { + %X = or i1 false, false + call void @bar(void()* @foo, i1 %X) + ret void + } + +In this example, "test" always passes ``@foo``/``false`` into ``bar``, which +ensures that it is dynamically called with the right calling conv (thus, the +code is perfectly well defined). If you run this through the inliner, you +get this (the explicit "or" is there so that the inliner doesn't dead code +eliminate a bunch of stuff): + +.. code-block:: llvm + + define fastcc void @foo() { + ret void + } + define void @test() { + %X = or i1 false, false + br i1 %X, label %T.i, label %F.i + T.i: + call void @foo() + br label %bar.exit + F.i: + call fastcc void @foo() + br label %bar.exit + bar.exit: + ret void + } + +Here you can see that the inlining pass made an undefined call to ``@foo`` +with the wrong calling convention. We really don't want to make the inliner +have to know about this sort of thing, so it needs to be valid code. In this +case, dead code elimination can trivially remove the undefined code. However, +if ``%X`` was an input argument to ``@test``, the inliner would produce this: + +.. code-block:: llvm + + define fastcc void @foo() { + ret void + } + + define void @test(i1 %X) { + br i1 %X, label %T.i, label %F.i + T.i: + call void @foo() + br label %bar.exit + F.i: + call fastcc void @foo() + br label %bar.exit + bar.exit: + ret void + } + +The interesting thing about this is that ``%X`` *must* be false for the +code to be well-defined, but no amount of dead code elimination will be able +to delete the broken call as unreachable. However, since +``instcombine``/``simplifycfg`` turns the undefined call into unreachable, we +end up with a branch on a condition that goes to unreachable: a branch to +unreachable can never happen, so "``-inline -instcombine -simplifycfg``" is +able to produce: + +.. code-block:: llvm + + define fastcc void @foo() { + ret void + } + define void @test(i1 %X) { + F.i: + call fastcc void @foo() + ret void + } diff --git a/docs/userguides.rst b/docs/userguides.rst index eb70028e7b..d59ef7f519 100644 --- a/docs/userguides.rst +++ b/docs/userguides.rst @@ -7,6 +7,7 @@ User Guides :hidden: CommandGuide/index + FAQ \ @@ -45,7 +46,7 @@ User Guides A list of optimizations and analyses implemented in LLVM. - * `Frequently Asked Questions <FAQ.html>`_ + * :ref:`faq` A list of common questions and problems and their solutions. |