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| author | mike-m <mikem.llvm@gmail.com> | 2010-05-06 23:45:43 +0000 |
|---|---|---|
| committer | mike-m <mikem.llvm@gmail.com> | 2010-05-06 23:45:43 +0000 |
| commit | 68cb31901c590cabceee6e6356d62c84142114cb (patch) | |
| tree | 6444bddc975b662fbe47d63cd98a7b776a407c1a /docs/FAQ.html | |
| parent | c26ae5ab7e2d65b67c97524e66f50ce86445dec7 (diff) | |
| download | external_llvm-68cb31901c590cabceee6e6356d62c84142114cb.zip external_llvm-68cb31901c590cabceee6e6356d62c84142114cb.tar.gz external_llvm-68cb31901c590cabceee6e6356d62c84142114cb.tar.bz2 | |
Overhauled llvm/clang docs builds. Closes PR6613.
NOTE: 2nd part changeset for cfe trunk to follow.
*** PRE-PATCH ISSUES ADDRESSED
- clang api docs fail build from objdir
- clang/llvm api docs collide in install PREFIX/
- clang/llvm main docs collide in install
- clang/llvm main docs have full of hard coded destination
assumptions and make use of absolute root in static html files;
namely CommandGuide tools hard codes a website destination
for cross references and some html cross references assume
website root paths
*** IMPROVEMENTS
- bumped Doxygen from 1.4.x -> 1.6.3
- splits llvm/clang docs into 'main' and 'api' (doxygen) build trees
- provide consistent, reliable doc builds for both main+api docs
- support buid vs. install vs. website intentions
- support objdir builds
- document targets with 'make help'
- correct clean and uninstall operations
- use recursive dir delete only where absolutely necessary
- added call function fn.RMRF which safeguards against botched 'rm -rf';
if any target (or any variable is evaluated) which attempts
to remove any dirs which match a hard-coded 'safelist', a verbose
error will be printed and make will error-stop.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103213 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'docs/FAQ.html')
| -rw-r--r-- | docs/FAQ.html | 938 |
1 files changed, 0 insertions, 938 deletions
diff --git a/docs/FAQ.html b/docs/FAQ.html deleted file mode 100644 index bf050c4..0000000 --- a/docs/FAQ.html +++ /dev/null @@ -1,938 +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("llvm.css"); - .question { font-weight: bold } - .answer { margin-left: 2em } - </style> -</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> - <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> |