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-<head>
- <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
- <title>LLVM: Frequently Asked Questions</title>
- <style type="text/css">
- @import url("llvm.css");
- .question { font-weight: bold }
- .answer { margin-left: 2em }
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-</head>
-<body>
-
-<div class="doc_title">
- LLVM: Frequently Asked Questions
-</div>
-
-<ol>
- <li><a href="#license">License</a>
- <ol>
- <li>Why are the LLVM source code and the front-end distributed under
- different licenses?</li>
-
- <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>When I use the test suite, all of the C Backend tests fail. What is
- wrong?</li>
-
- <li>After Subversion update, rebuilding gives the error "No rule to make
- target".</li>
-
- <li><a href="#llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
- work.</a></li>
-
- <li><a href="#srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir,
- it fails. Why?</a></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 GCC Front End</a>
- <ol>
- <li>When I compile software that uses a configure script, the configure
- script thinks my system has all of the header files and libraries it is
- testing for. How do I get configure to work correctly?</li>
-
- <li>When I compile code using the LLVM GCC front end, it complains that it
- cannot find libcrtend.a?</li>
-
- <li>How can I disable all optimizations when compiling code using the LLVM
- GCC front end?</li>
-
- <li><a href="#translatecxx">Can I use LLVM to convert C++ code to C
- code?</a></li>
-
- <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 GCC front-end</a>
- <ol>
- <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
- <tt>_GLOBAL__I__tmp_webcompile...</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>
-
-
-<!-- *********************************************************************** -->
-<div class="doc_section">
- <a name="license">License</a>
-</div>
-<!-- *********************************************************************** -->
-
-<div class="question">
-<p>Why are the LLVM source code and the front-end distributed under different
- licenses?</p>
-</div>
-
-<div class="answer">
-<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
- Our aim is to distribute LLVM source code under a <em>much less
- restrictive</em> license, in particular one that does not compel users who
- distribute tools based on modifying the source to redistribute the modified
- source code as well.</p>
-</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 class="doc_section">
- <a name="source">Source Code</a>
-</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 GCC front end code is not as portable as the LLVM suite, so it may not
- compile as well on unsupported platforms.</li>
-
- <li>The LLVM 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 class="doc_section">
- <a name="build">Build Problems</a>
-</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 linker 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 Borne 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 class="question">
-<p><a name="llvmc">The <tt>llvmc</tt> program gives me errors/doesn't
- work.</a></p>
-</div>
-
-<div class="answer">
-<p><tt>llvmc</tt> is experimental and isn't really supported. We suggest
- using <tt>llvm-gcc</tt> instead.</p>
-</div>
-
-<div class="question">
-<p><a name="srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir, it
- fails. Why?</a></p>
-</div>
-
-<div class="answer">
-<p>The <tt>GNUmakefile</tt> in the top-level directory of LLVM-GCC is a special
- <tt>Makefile</tt> used by Apple to invoke the <tt>build_gcc</tt> script after
- setting up a special environment. This has the unfortunate side-effect that
- trying to build LLVM-GCC with srcdir == objdir in a "non-Apple way" invokes
- the <tt>GNUmakefile</tt> instead of <tt>Makefile</tt>. Because the
- environment isn't set up correctly to do this, the build fails.</p>
-
-<p>People not building LLVM-GCC the "Apple way" need to build LLVM-GCC with
- srcdir != objdir, or simply remove the GNUmakefile entirely.</p>
-
-<p>We regret the inconvenience.</p>
-</div>
-
-<!-- *********************************************************************** -->
-<div class="doc_section"><a name="felangs">Source Languages</a></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 a special version of GCC that LLVM calls the
- <a href="#cfe">C Front End</a></p>
-
-<p>There is an incomplete version of a Java front end available in the
- <tt>java</tt> module. There is no documentation on this yet so you'll need to
- download the code, compile it, and try it.</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. There is, however, a <i>mostly
- implemented</i> configuration-driven
- <a href="CompilerDriver.html">compiler driver</a> which simplifies the task
- of running optimizations, linking, and executable generation.</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 class="doc_section">
- <a name="cfe">Using the GCC Front End</a>
-</div>
-
-<div class="question">
-<p>When I compile software that uses a configure script, the configure script
- thinks my system has all of the header files and libraries it is testing for.
- How do I get configure to work correctly?</p>
-</div>
-
-<div class="answer">
-<p>The configure script is getting things wrong because the LLVM linker allows
- symbols to be undefined at link time (so that they can be resolved during JIT
- or translation to the C back end). That is why configure thinks your system
- "has everything."</p>
-
-<p>To work around this, perform the following steps:</p>
-
-<ol>
- <li>Make sure the CC and CXX environment variables contains the full path to
- the LLVM GCC front end.</li>
-
- <li>Make sure that the regular C compiler is first in your PATH. </li>
-
- <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
-</ol>
-
-<p>This will allow the <tt>llvm-ld</tt> linker to create a native code
- executable instead of shell script that runs the JIT. Creating native code
- requires standard linkage, which in turn will allow the configure script to
- find out if code is not linking on your system because the feature isn't
- available on your system.</p>
-</div>
-
-<div class="question">
-<p>When I compile code using the LLVM GCC front end, it complains that it cannot
- find libcrtend.a.
-</p>
-</div>
-
-<div class="answer">
-<p>The only way this can happen is if you haven't installed the runtime
- library. To correct this, do:</p>
-
-<pre class="doc_code">
-% cd llvm/runtime
-% make clean ; make install-bytecode
-</pre>
-</div>
-
-<div class="question">
-<p>How can I disable all optimizations when compiling code using the LLVM GCC
- front end?</p>
-</div>
-
-<div class="answer">
-<p>Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
- optimizations done at the llvm level, leaving you with the truly horrible
- code that you desire.</p>
-</div>
-
-
-<div class="question">
-<p><a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a></p>
-</div>
-
-<div class="answer">
-<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
- Note that the generated C code will be very low level (all loops are lowered
- to gotos, etc) and not very pretty (comments are stripped, original source
- formatting is totally lost, variables are renamed, expressions are
- regrouped), so this may not be what you're looking for. Also, there are
- several limitations noted below.<p>
-
-<p>Use commands like this:</p>
-
-<ol>
- <li><p>Compile your program with llvm-g++:</p>
-
-<pre class="doc_code">
-% llvm-g++ -emit-llvm x.cpp -o program.bc -c
-</pre>
-
- <p>or:</p>
-
-<pre class="doc_code">
-% llvm-g++ a.cpp -c -emit-llvm
-% llvm-g++ b.cpp -c -emit-llvm
-% llvm-ld a.o b.o -o program
-</pre>
-
- <p>This will generate program and program.bc. The .bc
- file is the LLVM version of the program all linked together.</p></li>
-
- <li><p>Convert the LLVM code to C code, using the LLC tool with the C
- backend:</p>
-
-<pre class="doc_code">
-% llc -march=c program.bc -o program.c
-</pre></li>
-
- <li><p>Finally, compile the C file:</p>
-
-<pre class="doc_code">
-% cc x.c -lstdc++
-</pre></li>
-
-</ol>
-
-<p>Using LLVM does not eliminate the need for C++ library support. If you use
- the llvm-g++ front-end, the generated code will depend on g++'s C++ support
- libraries in the same way that code generated from g++ would. If you use
- another C++ front-end, the generated code will depend on whatever library
- that front-end would normally require.</p>
-
-<p>If you are working on a platform that does not provide any C++ libraries, you
- may be able to manually compile libstdc++ to LLVM bitcode, statically link it
- into your program, then use the commands above to convert the whole result
- into C code. Alternatively, you might compile the libraries and your
- application into two different chunks of C code and link them.</p>
-
-<p>Note that, by default, the C back end does not support exception handling.
- If you want/need it for a certain program, you can enable it by passing
- "-enable-correct-eh-support" to the llc program. The resultant code will use
- setjmp/longjmp to implement exception support that is relatively slow, and
- not C++-ABI-conforming on most platforms, but otherwise correct.</p>
-
-<p>Also, there are a number of other limitations of the C backend that cause it
- to produce code that does not fully conform to the C++ ABI on most
- platforms. Some of the C++ programs in LLVM's test suite are known to fail
- when compiled with the C back end because of ABI incompatibilities with
- standard C++ libraries.</p>
-</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 class="doc_section">
- <a name="cfe_code">Questions about code generated by the GCC front-end</a>
-</div>
-
-<div class="question">
-<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
- <tt>_GLOBAL__I__tmp_webcompile...</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>
-
-<!-- *********************************************************************** -->
-
-<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>
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- 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>
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projects / oota-llvm.git / blobdiff