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diff --git a/release_23/docs/FAQ.html b/release_23/docs/FAQ.html deleted file mode 100644 index 95e425aac669..000000000000 --- a/release_23/docs/FAQ.html +++ /dev/null @@ -1,737 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> -<head> - <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>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> - </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></div> - <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="#translatec++">Can I use LLVM to convert C++ code to C code?</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> - </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/releases/1.3/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> - -<div class="doc_code"> -<pre> -% PATH=[the path without the bad program] ./configure ... -</pre> -</div> - - <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> - -<div class="doc_code"> -<pre>% ./config.status <relative path to Makefile></pre> -</div> - -<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> - -<div class="doc_code"> -<pre>% gmake ENABLE_PROFILING=1</pre> -</div> - -<p>...then you must run the tests with the following commands:</p> - -<div class="doc_code"> -<pre> -% cd llvm/test -% gmake ENABLE_PROFILING=1 -</pre> -</div> - -</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/PR?13392">a bug in GCC</a>, and -affects projects other than LLVM. Try upgrading or downgrading your GCC.</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> - -<div class="doc_code"> -<pre> -gmake[2]: *** No rule to make target `/path/to/somefile', needed by -`/path/to/another/file.d'.<br> -Stop. -</pre> -</div> - -<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> - -<div class="doc_code"> -<pre> -% cd $LLVM_OBJ_DIR -% rm -f `find . -name \*\.d` -% gmake -</pre> -</div> - -<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="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>In the <tt>stacker</tt> module is a compiler and runtime - library for the Stacker language, a "toy" language loosely based on Forth.</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="langhlsupp"> - 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> - -<div class="doc_code"> -<pre> -% cd llvm/runtime -% make clean ; make install-bytecode -</pre> -</div> -</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="translatec++">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. However, this is a good way to add -C++ support for a processor that does not otherwise have a C++ compiler. -</p> - -<p>Use commands like this:</p> - -<ol> - <li><p>Compile your program as normal with llvm-g++:</p> - -<div class="doc_code"> -<pre> -% llvm-g++ x.cpp -o program -</pre> -</div> - - <p>or:</p> - -<div class="doc_code"> -<pre> -% llvm-g++ a.cpp -c -% llvm-g++ b.cpp -c -% llvm-g++ a.o b.o -o program -</pre> -</div> - - <p>With llvm-gcc3, 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> - -<div class="doc_code"> -<pre> -% llc -march=c program.bc -o program.c -</pre> -</div></li> - -<li><p>Finally, compile the C file:</p> - -<div class="doc_code"> -<pre> -% cc x.c -</pre> -</div></li> - -</ol> - -<p>Note that, by default, the C backend 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 correct but relatively -slow.</p> - -<p>Also note: this specific sequence of commands won't work if you use a -function defined in the C++ runtime library (or any other C++ library). To -access an external C++ library, you must 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 can compile the -libraries and your application into two different chunks of C code and link -them.</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"> - <a name="cfe_code">Questions about code generated by the GCC front-end</a> -</div> - -<div class="question"> -<a name="iosinit"></a> -<p> What is this <tt>llvm.global_ctors</tt> and -<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include -<iostream>?</p> -</div> - -<div class="answer"> - -<p>If you #include the <iostream> 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"></a> -Where did all of my code go?? -</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"></a> -<p>What is this "<tt>undef</tt>" thing that shows up in my code? -</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> - -<div class="doc_code"> -<pre> -int X() { int i; return i; } -</pre> -</div> - -<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has -a value specified for it.</p> -</div> - -<!-- *********************************************************************** --> - -<hr> -<address> - <a href="http://jigsaw.w3.org/css-validator/check/referer"><img - src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a> - <a href="http://validator.w3.org/check/referer"><img - src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a> - - <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br> - Last modified: $Date$ -</address> - -</body> -</html> |