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| author | mike-m <mikem.llvm@gmail.com> | 2010-05-07 00:28:04 +0000 |
|---|---|---|
| committer | mike-m <mikem.llvm@gmail.com> | 2010-05-07 00:28:04 +0000 |
| commit | e2c3a49c8029ebd9ef530101cc24c66562e3dff5 (patch) | |
| tree | 91bf9600cc8df90cf99751a8f8bafc317cffc91e /docs/FAQ.html | |
| parent | c10b5afbe8138b0fdf3af4ed3e1ddf96cf3cb4cb (diff) | |
| download | external_llvm-e2c3a49c8029ebd9ef530101cc24c66562e3dff5.zip external_llvm-e2c3a49c8029ebd9ef530101cc24c66562e3dff5.tar.gz external_llvm-e2c3a49c8029ebd9ef530101cc24c66562e3dff5.tar.bz2 | |
Revert r103213. It broke several sections of live website.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103219 91177308-0d34-0410-b5e6-96231b3b80d8
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diff --git a/docs/FAQ.html b/docs/FAQ.html new file mode 100644 index 0000000..bf050c4 --- /dev/null +++ b/docs/FAQ.html @@ -0,0 +1,938 @@ +<!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> |