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[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
This commit is contained in:
Michael J. Spencer
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|
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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||||
<title>LLVM: Frequently Asked Questions</title>
|
||||
<style type="text/css">
|
||||
@import url("_static/llvm.css");
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||||
.question { font-weight: bold }
|
||||
.answer { margin-left: 2em }
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||||
</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>
|
||||
464
docs/FAQ.rst
Normal file
464
docs/FAQ.rst
Normal file
@ -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
|
||||
}
|
||||
@ -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.
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user