[llvm-commits] [llvm] r158677 - in /llvm/trunk/docs: FAQ.html FAQ.rst userguides.rst

Michael J. Spencer bigcheesegs at gmail.com
Mon Jun 18 13:21:38 PDT 2012


Author: mspencer
Date: Mon Jun 18 15:21:38 2012
New Revision: 158677

URL: http://llvm.org/viewvc/llvm-project?rev=158677&view=rev
Log:
[docs] Port FAQ over to Sphinx.

Patch by Mikael Lyngvig!

Added:
    llvm/trunk/docs/FAQ.rst
Removed:
    llvm/trunk/docs/FAQ.html
Modified:
    llvm/trunk/docs/userguides.rst

Removed: llvm/trunk/docs/FAQ.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/FAQ.html?rev=158676&view=auto
==============================================================================
--- llvm/trunk/docs/FAQ.html (original)
+++ llvm/trunk/docs/FAQ.html (removed)
@@ -1,758 +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("_static/llvm.css");
-    .question { font-weight: bold }
-    .answer   { margin-left: 2em  }
-  </style>
-</head>
-<body>
-
-<h1>
-  LLVM: Frequently Asked Questions
-</h1>
-
-<ol>
-  <li><a href="#license">License</a>
-  <ol>
-    <li>Does the University of Illinois Open Source License really qualify as an
-       "open source" license?</li>
-
-    <li>Can I modify LLVM source code and redistribute the modified source?</li>
-
-    <li>Can I modify LLVM source code and redistribute binaries or other tools
-        based on it, without redistributing the source?</li>
-  </ol></li>
-
-  <li><a href="#source">Source code</a>
-  <ol>
-    <li>In what language is LLVM written?</li>
-
-    <li>How portable is the LLVM source code?</li>
-  </ol></li>
-
-  <li><a href="#build">Build Problems</a>
-  <ol>
-    <li>When I run configure, it finds the wrong C compiler.</li>
-
-    <li>The <tt>configure</tt> script finds the right C compiler, but it uses
-        the LLVM linker from a previous build.  What do I do?</li>
-
-    <li>When creating a dynamic library, I get a strange GLIBC error.</li>
-
-    <li>I've updated my source tree from Subversion, and now my build is trying
-        to use a file/directory that doesn't exist.</li>
-
-    <li>I've modified a Makefile in my source tree, but my build tree keeps
-        using the old version.  What do I do?</li>
-
-    <li>I've upgraded to a new version of LLVM, and I get strange build
-        errors.</li>
-
-    <li>I've built LLVM and am testing it, but the tests freeze.</li>
-
-    <li>Why do test results differ when I perform different types of
-        builds?</li>
-
-    <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
-
-    <li>Compiling LLVM with GCC succeeds, but the resulting tools do not work,
-        what can be wrong?</li>
-
-    <li>After Subversion update, rebuilding gives the error "No rule to make
-        target".</li>
-
-  </ol></li>
-
-  <li><a href="#felangs">Source Languages</a>
-  <ol>
-    <li><a href="#langs">What source languages are supported?</a></li>
-
-    <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
-        should I interface with the LLVM middle-end optimizers and back-end code
-        generators?</a></li>
-
-    <li><a href="#langhlsupp">What support is there for higher level source
-        language constructs for building a compiler?</a></li>
-
-    <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
-      instruction. Help!</a></li>
-  </ol>
-
-  <li><a href="#cfe">Using the C and C++ Front Ends</a>
-  <ol>
-    <li><a href="#platformindependent">Can I compile C or C++ code to
-        platform-independent LLVM bitcode?</a></li>
-  </ol>
-  </li>
-
-  <li><a href="#cfe_code">Questions about code generated by the demo page</a>
-  <ol>
-     <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
-          <tt>_GLOBAL__I_a...</tt> stuff that happens when I
-          #include <iostream>?</a></li>
-
-     <li><a href="#codedce">Where did all of my code go??</a></li>
-
-     <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in
-         my code?</a></li>
-         
-      <li><a href="#callconvwrong">Why does instcombine + simplifycfg turn
-   a call to a function with a mismatched calling convention into "unreachable"?
-   Why not make the verifier reject it?</a></li>
-  </ol>
-  </li>
-</ol>
-
-<div class="doc_author">
-  <p>Written by <a href="http://llvm.org/">The LLVM Team</a></p>
-</div>
-
-
-<!-- *********************************************************************** -->
-<h2>
-  <a name="license">License</a>
-</h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<div class="question">
-<p>Does the University of Illinois Open Source License really qualify as an
-   "open source" license?</p>
-</div>
-
-<div class="answer">
-<p>Yes, the license
-   is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by
-   the Open Source Initiative (OSI).</p>
-</div>
-
-<div class="question">
-<p>Can I modify LLVM source code and redistribute the modified source?</p>
-</div>
-
-<div class="answer">
-<p>Yes.  The modified source distribution must retain the copyright notice and
-   follow the three bulletted conditions listed in
-   the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM
-   license</a>.</p>
-</div>
-
-<div class="question">
-<p>Can I modify LLVM source code and redistribute binaries or other tools based
-   on it, without redistributing the source?</p>
-</div>
-
-<div class="answer">
-<p>Yes. This is why we distribute LLVM under a less restrictive license than
-   GPL, as explained in the first question above.</p>
-</div>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2>
-  <a name="source">Source Code</a>
-</h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<div class="question">
-<p>In what language is LLVM written?</p>
-</div>
-
-<div class="answer">
-<p>All of the LLVM tools and libraries are written in C++ with extensive use of
-   the STL.</p>
-</div>
-
-<div class="question">
-<p>How portable is the LLVM source code?</p>
-</div>
-
-<div class="answer">
-<p>The LLVM source code should be portable to most modern UNIX-like operating
-systems.  Most of the code is written in standard C++ with operating system
-services abstracted to a support library.  The tools required to build and test
-LLVM have been ported to a plethora of platforms.</p>
-
-<p>Some porting problems may exist in the following areas:</p>
-
-<ul>
-  <li>The autoconf/makefile build system relies heavily on UNIX shell tools,
-      like the Bourne Shell and sed.  Porting to systems without these tools
-      (MacOS 9, Plan 9) Will require more effort.</li>
-</ul>
-
-</div>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2>
-  <a name="build">Build Problems</a>
-</h2>
-<!-- *********************************************************************** -->
-
-<div>
-
-<div class="question">
-<p>When I run configure, it finds the wrong C compiler.</p>
-</div>
-
-<div class="answer">
-<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
-   <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt>
-   and <tt>CXX</tt> for the C and C++ compiler, respectively.</p>
-
-<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
-   <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
-   explicitly.</p>
-
-</div>
-
-<div class="question">
-<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
-   LLVM tools from a previous build.  What do I do?</p>
-</div>
-
-<div class="answer">
-<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
-   if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
-   it:</p>
-
-<ol>
-  <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
-      program appears first in the <tt>PATH</tt>.  This may work, but may not be
-      convenient when you want them <i>first</i> in your path for other
-      work.</p></li>
-
-  <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
-      correct. In a Bourne compatible shell, the syntax would be:</p>
-
-<pre class="doc_code">
-% PATH=[the path without the bad program] ./configure ...
-</pre>
-
-      <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
-         to do its work without having to adjust your <tt>PATH</tt>
-         permanently.</p></li>
-</ol>
-</div>
-
-<div class="question">
-<p>When creating a dynamic library, I get a strange GLIBC error.</p>
-</div>
-
-<div class="answer">
-<p>Under some operating systems (i.e. Linux), libtool does not work correctly if
-   GCC was compiled with the --disable-shared option.  To work around this,
-   install your own version of GCC that has shared libraries enabled by
-   default.</p>
-</div>
-
-<div class="question">
-<p>I've updated my source tree from Subversion, and now my build is trying to
-   use a file/directory that doesn't exist.</p>
-</div>
-
-<div class="answer">
-<p>You need to re-run configure in your object directory.  When new Makefiles
-   are added to the source tree, they have to be copied over to the object tree
-   in order to be used by the build.</p>
-</div>
-
-<div class="question">
-<p>I've modified a Makefile in my source tree, but my build tree keeps using the
-   old version.  What do I do?</p>
-</div>
-
-<div class="answer">
-<p>If the Makefile already exists in your object tree, you can just run the
-   following command in the top level directory of your object tree:</p>
-
-<pre class="doc_code">
-% ./config.status <relative path to Makefile>
-</pre>
-
-<p>If the Makefile is new, you will have to modify the configure script to copy
-   it over.</p>
-</div>
-
-<div class="question">
-<p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
-</div>
-
-<div class="answer">
-
-<p>Sometimes, changes to the LLVM source code alters how the build system works.
-   Changes in libtool, autoconf, or header file dependencies are especially
-   prone to this sort of problem.</p>
-
-<p>The best thing to try is to remove the old files and re-build.  In most
-   cases, this takes care of the problem.  To do this, just type <tt>make
-   clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
-</div>
-
-<div class="question">
-<p>I've built LLVM and am testing it, but the tests freeze.</p>
-</div>
-
-<div class="answer">
-<p>This is most likely occurring because you built a profile or release
-   (optimized) build of LLVM and have not specified the same information on the
-   <tt>gmake</tt> command line.</p>
-
-<p>For example, if you built LLVM with the command:</p>
-
-<pre class="doc_code">
-% gmake ENABLE_PROFILING=1
-</pre>
-
-<p>...then you must run the tests with the following commands:</p>
-
-<pre class="doc_code">
-% cd llvm/test
-% gmake ENABLE_PROFILING=1
-</pre>
-</div>
-
-<div class="question">
-<p>Why do test results differ when I perform different types of builds?</p>
-</div>
-
-<div class="answer">
-<p>The LLVM test suite is dependent upon several features of the LLVM tools and
-   libraries.</p>
-
-<p>First, the debugging assertions in code are not enabled in optimized or
-   profiling builds.  Hence, tests that used to fail may pass.</p>
-	
-<p>Second, some tests may rely upon debugging options or behavior that is only
-   available in the debug build.  These tests will fail in an optimized or
-   profile build.</p>
-</div>
-
-<div class="question">
-<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
-</div>
-
-<div class="answer">
-<p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in
-   GCC</a>, and affects projects other than LLVM.  Try upgrading or downgrading
-   your GCC.</p>
-</div>
-
-<div class="question">
-<p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what
-   can be wrong?</p>
-</div>
-
-<div class="answer">
-<p>Several versions of GCC have shown a weakness in miscompiling the LLVM
-   codebase. Please consult your compiler version (<tt>gcc --version</tt>) to
-   find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>.
-   If so, your only option is to upgrade GCC to a known good version.</p>
-</div>
-
-<div class="question">
-<p>After Subversion update, rebuilding gives the error "No rule to make
-   target".</p>
-</div>
-
-<div class="answer">
-<p>If the error is of the form:</p>
-
-<pre class="doc_code">
-gmake[2]: *** No rule to make target `/path/to/somefile', needed by
-`/path/to/another/file.d'.<br>
-Stop.
-</pre>
-
-<p>This may occur anytime files are moved within the Subversion repository or
-   removed entirely.  In this case, the best solution is to erase all
-   <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
-
-<pre class="doc_code">
-% cd $LLVM_OBJ_DIR
-% rm -f `find . -name \*\.d` 
-% gmake 
-</pre>
-
-<p>In other cases, it may be necessary to run <tt>make clean</tt> before
-   rebuilding.</p>
-</div>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2>
-  <a name="felangs">Source Languages</a>
-</h2>
-
-<div>
-
-<div class="question">
-<p><a name="langs">What source languages are supported?</a></p>
-</div>
-
-<div class="answer">
-<p>LLVM currently has full support for C and C++ source languages. These are
-   available through both <a href="http://clang.llvm.org/">Clang</a> and
-   <a href="http://dragonegg.llvm.org/">DragonEgg</a>.</p>
-
-<p>The PyPy developers are working on integrating LLVM into the PyPy backend so
-   that PyPy language can translate to LLVM.</p>
-</div>
-
-<div class="question">
-<p><a name="langirgen">I'd like to write a self-hosting LLVM compiler. How
-   should I interface with the LLVM middle-end optimizers and back-end code
-   generators?</a></p>
-</div>
-
-<div class="answer">
-<p>Your compiler front-end will communicate with LLVM by creating a module in
-   the LLVM intermediate representation (IR) format. Assuming you want to write
-   your language's compiler in the language itself (rather than C++), there are
-   3 major ways to tackle generating LLVM IR from a front-end:</p>
-
-<ul>
-  <li><strong>Call into the LLVM libraries code using your language's FFI
-      (foreign function interface).</strong>
-
-    <ul>
-      <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc
-          format</li>
-
-      <li><em>for:</em> enables running LLVM optimization passes without a
-          emit/parse overhead</li>
-
-      <li><em>for:</em> adapts well to a JIT context</li>
-
-      <li><em>against:</em> lots of ugly glue code to write</li>
-    </ul></li>
-
-  <li>  <strong>Emit LLVM assembly from your compiler's native language.</strong>
-    <ul>
-      <li><em>for:</em> very straightforward to get started</li>
-
-      <li><em>against:</em> the .ll parser is slower than the bitcode reader
-          when interfacing to the middle end</li>
-
-      <li><em>against:</em> you'll have to re-engineer the LLVM IR object model
-          and asm writer in your language</li>
-
-      <li><em>against:</em> it may be harder to track changes to the IR</li>
-    </ul></li>
-
-  <li><strong>Emit LLVM bitcode from your compiler's native language.</strong>
-
-    <ul>
-      <li><em>for:</em> can use the more-efficient bitcode reader when
-          interfacing to the middle end</li>
-
-      <li><em>against:</em> you'll have to re-engineer the LLVM IR object 
-          model and bitcode writer in your language</li>
-
-      <li><em>against:</em> it may be harder to track changes to the IR</li>
-    </ul></li>
-</ul>
-
-<p>If you go with the first option, the C bindings in include/llvm-c should help
-   a lot, since most languages have strong support for interfacing with C. The
-   most common hurdle with calling C from managed code is interfacing with the
-   garbage collector. The C interface was designed to require very little memory
-   management, and so is straightforward in this regard.</p>
-</div>
-
-<div class="question">
-<p><a name="langhlsupp">What support is there for a higher level source language
-   constructs for building a compiler?</a></p>
-</div>
-
-<div class="answer">
-<p>Currently, there isn't much. LLVM supports an intermediate representation
-   which is useful for code representation but will not support the high level
-   (abstract syntax tree) representation needed by most compilers. There are no
-   facilities for lexical nor semantic analysis.</p>
-</div>
-
-<div class="question">
-<p><a name="getelementptr">I don't understand the GetElementPtr
-   instruction. Help!</a></p>
-</div>
-
-<div class="answer">
-<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
-   Instruction</a>.</p>
-</div>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2>
-  <a name="cfe">Using the C and C++ Front Ends</a>
-</h2>
-
-<div>
-
-<div class="question">
-<p><a name="platformindependent">Can I compile C or C++ code to
-   platform-independent LLVM bitcode?</a></p>
-</div>
-
-<div class="answer">
-<p>No. C and C++ are inherently platform-dependent languages. The most obvious
-   example of this is the preprocessor. A very common way that C code is made
-   portable is by using the preprocessor to include platform-specific code. In
-   practice, information about other platforms is lost after preprocessing, so
-   the result is inherently dependent on the platform that the preprocessing was
-   targeting.</p>
-
-<p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to
-   vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to
-   a constant immediately, thus hard-wiring a platform-specific detail.</p>
-
-<p>Also, since many platforms define their ABIs in terms of C, and since LLVM is
-   lower-level than C, front-ends currently must emit platform-specific IR in
-   order to have the result conform to the platform ABI.</p>
-</div>
-
-</div>
-
-<!-- *********************************************************************** -->
-<h2>
-  <a name="cfe_code">Questions about code generated by the demo page</a>
-</h2>
-
-<div>
-
-<div class="question">
-<p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
-   <tt>_GLOBAL__I_a...</tt> stuff that happens when I <tt>#include
-   <iostream></tt>?</a></p>
-</div>
-
-<div class="answer">
-<p>If you <tt>#include</tt> the <tt><iostream></tt> header into a C++
-   translation unit, the file will probably use
-   the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects.  However, C++
-   does not guarantee an order of initialization between static objects in
-   different translation units, so if a static ctor/dtor in your .cpp file
-   used <tt>std::cout</tt>, for example, the object would not necessarily be
-   automatically initialized before your use.</p>
-
-<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
-   STL that we use declares a static object that gets created in every
-   translation unit that includes <tt><iostream></tt>.  This object has a
-   static constructor and destructor that initializes and destroys the global
-   iostream objects before they could possibly be used in the file.  The code
-   that you see in the .ll file corresponds to the constructor and destructor
-   registration code.
-</p>
-
-<p>If you would like to make it easier to <b>understand</b> the LLVM code
-   generated by the compiler in the demo page, consider using <tt>printf()</tt>
-   instead of <tt>iostream</tt>s to print values.</p>
-</div>
-
-<!--=========================================================================-->
-
-<div class="question">
-<p><a name="codedce">Where did all of my code go??</a></p>
-</div>
-
-<div class="answer">
-<p>If you are using the LLVM demo page, you may often wonder what happened to
-   all of the code that you typed in.  Remember that the demo script is running
-   the code through the LLVM optimizers, so if your code doesn't actually do
-   anything useful, it might all be deleted.</p>
-
-<p>To prevent this, make sure that the code is actually needed.  For example, if
-   you are computing some expression, return the value from the function instead
-   of leaving it in a local variable.  If you really want to constrain the
-   optimizer, you can read from and assign to <tt>volatile</tt> global
-   variables.</p>
-</div>
-
-<!--=========================================================================-->
-
-<div class="question">
-<p><a name="undef">What is this "<tt>undef</tt>" thing that shows up in my
-   code?</a></p>
-</div>
-
-<div class="answer">
-<p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of
-   representing a value that is not defined.  You can get these if you do not
-   initialize a variable before you use it.  For example, the C function:</p>
-
-<pre class="doc_code">
-int X() { int i; return i; }
-</pre>
-
-<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
-   value specified for it.</p>
-</div>
-
-<!--=========================================================================-->
-
-<div class="question">
-<p><a name="callconvwrong">Why does instcombine + simplifycfg turn
-   a call to a function with a mismatched calling convention into "unreachable"?
-   Why not make the verifier reject it?</a></p>
-</div>
-
-<div class="answer">
-<p>This is a common problem run into by authors of front-ends that are using
-custom calling conventions: you need to make sure to set the right calling
-convention on both the function and on each call to the function.  For example,
-this code:</p>
-
-<pre class="doc_code">
-define fastcc void @foo() {
-        ret void
-}
-define void @bar() {
-        call void @foo()
-        ret void
-}
-</pre>
-
-<p>Is optimized to:</p>
-
-<pre class="doc_code">
-define fastcc void @foo() {
-	ret void
-}
-define void @bar() {
-	unreachable
-}
-</pre>
-
-<p>... with "opt -instcombine -simplifycfg".  This often bites people because
-"all their code disappears".  Setting the calling convention on the caller and
-callee is required for indirect calls to work, so people often ask why not make
-the verifier reject this sort of thing.</p>
-
-<p>The answer is that this code has undefined behavior, but it is not illegal.
-If we made it illegal, then every transformation that could potentially create
-this would have to ensure that it doesn't, and there is valid code that can
-create this sort of construct (in dead code).  The sorts of things that can
-cause this to happen are fairly contrived, but we still need to accept them.
-Here's an example:</p>
-
-<pre class="doc_code">
-define fastcc void @foo() {
-        ret void
-}
-define internal void @bar(void()* %FP, i1 %cond) {
-        br i1 %cond, label %T, label %F
-T:  
-        call void %FP()
-        ret void
-F:
-        call fastcc void %FP()
-        ret void
-}
-define void @test() {
-        %X = or i1 false, false
-        call void @bar(void()* @foo, i1 %X)
-        ret void
-} 
-</pre>
-
-<p>In this example, "test" always passes @foo/false into bar, which ensures that
-   it is dynamically called with the right calling conv (thus, the code is
-   perfectly well defined).  If you run this through the inliner, you get this
-   (the explicit "or" is there so that the inliner doesn't dead code eliminate
-   a bunch of stuff):
-</p>
-
-<pre class="doc_code">
-define fastcc void @foo() {
-	ret void
-}
-define void @test() {
-	%X = or i1 false, false
-	br i1 %X, label %T.i, label %F.i
-T.i:
-	call void @foo()
-	br label %bar.exit
-F.i:
-	call fastcc void @foo()
-	br label %bar.exit
-bar.exit:
-	ret void
-}
-</pre>
-
-<p>Here you can see that the inlining pass made an undefined call to @foo with
-  the wrong calling convention.  We really don't want to make the inliner have
-  to know about this sort of thing, so it needs to be valid code.  In this case,
-  dead code elimination can trivially remove the undefined code.  However, if %X
-  was an input argument to @test, the inliner would produce this:
-</p>
-
-<pre class="doc_code">
-define fastcc void @foo() {
-	ret void
-}
-
-define void @test(i1 %X) {
-	br i1 %X, label %T.i, label %F.i
-T.i:
-	call void @foo()
-	br label %bar.exit
-F.i:
-	call fastcc void @foo()
-	br label %bar.exit
-bar.exit:
-	ret void
-}
-</pre>
-
-<p>The interesting thing about this is that %X <em>must</em> be false for the
-code to be well-defined, but no amount of dead code elimination will be able to
-delete the broken call as unreachable.  However, since instcombine/simplifycfg
-turns the undefined call into unreachable, we end up with a branch on a
-condition that goes to unreachable: a branch to unreachable can never happen, so
-"-inline -instcombine -simplifycfg" is able to produce:</p>
-
-<pre class="doc_code">
-define fastcc void @foo() {
-	ret void
-}
-define void @test(i1 %X) {
-F.i:
-	call fastcc void @foo()
-	ret void
-}
-</pre>
-
-</div>
-
-</div>
-
-<!-- *********************************************************************** -->
-
-<hr>
-<address>
-  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
-  src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
-  <a href="http://validator.w3.org/check/referer"><img
-  src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
-
-  <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
-  Last modified: $Date$
-</address>
-
-</body>
-</html>

Added: llvm/trunk/docs/FAQ.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/FAQ.rst?rev=158677&view=auto
==============================================================================
--- llvm/trunk/docs/FAQ.rst (added)
+++ llvm/trunk/docs/FAQ.rst Mon Jun 18 15:21:38 2012
@@ -0,0 +1,464 @@
+.. _faq:
+
+================================
+Frequently Asked Questions (FAQ)
+================================
+
+.. contents::
+   :local:
+
+
+License
+=======
+
+Does the University of Illinois Open Source License really qualify as an "open source" license?
+-----------------------------------------------------------------------------------------------
+Yes, the license is `certified
+<http://www.opensource.org/licenses/UoI-NCSA.php>`_ by the Open Source
+Initiative (OSI).
+
+
+Can I modify LLVM source code and redistribute the modified source?
+-------------------------------------------------------------------
+Yes.  The modified source distribution must retain the copyright notice and
+follow the three bulletted conditions listed in the `LLVM license
+<http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT>`_.
+
+
+Can I modify the LLVM source code and redistribute binaries or other tools based on it, without redistributing the source?
+--------------------------------------------------------------------------------------------------------------------------
+Yes. This is why we distribute LLVM under a less restrictive license than GPL,
+as explained in the first question above.
+
+
+Source Code
+===========
+
+In what language is LLVM written?
+---------------------------------
+All of the LLVM tools and libraries are written in C++ with extensive use of
+the STL.
+
+
+How portable is the LLVM source code?
+-------------------------------------
+The LLVM source code should be portable to most modern Unix-like operating
+systems.  Most of the code is written in standard C++ with operating system
+services abstracted to a support library.  The tools required to build and
+test LLVM have been ported to a plethora of platforms.
+
+Some porting problems may exist in the following areas:
+
+* The autoconf/makefile build system relies heavily on UNIX shell tools,
+  like the Bourne Shell and sed.  Porting to systems without these tools
+  (MacOS 9, Plan 9) will require more effort.
+
+
+Build Problems
+==============
+
+When I run configure, it finds the wrong C compiler.
+----------------------------------------------------
+The ``configure`` script attempts to locate first ``gcc`` and then ``cc``,
+unless it finds compiler paths set in ``CC`` and ``CXX`` for the C and C++
+compiler, respectively.
+
+If ``configure`` finds the wrong compiler, either adjust your ``PATH``
+environment variable or set ``CC`` and ``CXX`` explicitly.
+
+
+The ``configure`` script finds the right C compiler, but it uses the LLVM tools from a previous build.  What do I do?
+---------------------------------------------------------------------------------------------------------------------
+The ``configure`` script uses the ``PATH`` to find executables, so if it's
+grabbing the wrong linker/assembler/etc, there are two ways to fix it:
+
+#. Adjust your ``PATH`` environment variable so that the correct program
+   appears first in the ``PATH``.  This may work, but may not be convenient
+   when you want them *first* in your path for other work.
+
+#. Run ``configure`` with an alternative ``PATH`` that is correct. In a
+   Bourne compatible shell, the syntax would be:
+
+.. code-block:: bash
+
+   % PATH=[the path without the bad program] ./configure ...
+
+This is still somewhat inconvenient, but it allows ``configure`` to do its
+work without having to adjust your ``PATH`` permanently.
+
+
+When creating a dynamic library, I get a strange GLIBC error.
+-------------------------------------------------------------
+Under some operating systems (i.e. Linux), libtool does not work correctly if
+GCC was compiled with the ``--disable-shared option``.  To work around this,
+install your own version of GCC that has shared libraries enabled by default.
+
+
+I've updated my source tree from Subversion, and now my build is trying to use a file/directory that doesn't exist.
+-------------------------------------------------------------------------------------------------------------------
+You need to re-run configure in your object directory.  When new Makefiles
+are added to the source tree, they have to be copied over to the object tree
+in order to be used by the build.
+
+
+I've modified a Makefile in my source tree, but my build tree keeps using the old version.  What do I do?
+---------------------------------------------------------------------------------------------------------
+If the Makefile already exists in your object tree, you can just run the
+following command in the top level directory of your object tree:
+
+.. code-block:: bash
+
+   % ./config.status <relative path to Makefile>;
+
+If the Makefile is new, you will have to modify the configure script to copy
+it over.
+
+
+I've upgraded to a new version of LLVM, and I get strange build errors.
+-----------------------------------------------------------------------
+Sometimes, changes to the LLVM source code alters how the build system works.
+Changes in ``libtool``, ``autoconf``, or header file dependencies are
+especially prone to this sort of problem.
+
+The best thing to try is to remove the old files and re-build.  In most cases,
+this takes care of the problem.  To do this, just type ``make clean`` and then
+``make`` in the directory that fails to build.
+
+
+I've built LLVM and am testing it, but the tests freeze.
+--------------------------------------------------------
+This is most likely occurring because you built a profile or release
+(optimized) build of LLVM and have not specified the same information on the
+``gmake`` command line.
+
+For example, if you built LLVM with the command:
+
+.. code-block:: bash
+
+   % gmake ENABLE_PROFILING=1
+
+...then you must run the tests with the following commands:
+
+.. code-block:: bash
+
+   % cd llvm/test
+   % gmake ENABLE_PROFILING=1
+
+Why do test results differ when I perform different types of builds?
+--------------------------------------------------------------------
+The LLVM test suite is dependent upon several features of the LLVM tools and
+libraries.
+
+First, the debugging assertions in code are not enabled in optimized or
+profiling builds.  Hence, tests that used to fail may pass.
+
+Second, some tests may rely upon debugging options or behavior that is only
+available in the debug build.  These tests will fail in an optimized or
+profile build.
+
+
+Compiling LLVM with GCC 3.3.2 fails, what should I do?
+------------------------------------------------------
+This is `a bug in GCC <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392>`_,
+and affects projects other than LLVM.  Try upgrading or downgrading your GCC.
+
+
+Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong?
+-----------------------------------------------------------------------------------------
+Several versions of GCC have shown a weakness in miscompiling the LLVM
+codebase.  Please consult your compiler version (``gcc --version``) to find
+out whether it is `broken <GettingStarted.html#brokengcc>`_.  If so, your only
+option is to upgrade GCC to a known good version.
+
+
+After Subversion update, rebuilding gives the error "No rule to make target".
+-----------------------------------------------------------------------------
+If the error is of the form:
+
+.. code-block:: bash
+
+   gmake[2]: *** No rule to make target `/path/to/somefile',
+   needed by `/path/to/another/file.d'.
+   Stop.
+
+This may occur anytime files are moved within the Subversion repository or
+removed entirely.  In this case, the best solution is to erase all ``.d``
+files, which list dependencies for source files, and rebuild:
+
+.. code-block:: bash
+
+   % cd $LLVM_OBJ_DIR
+   % rm -f `find . -name \*\.d`
+   % gmake
+
+In other cases, it may be necessary to run ``make clean`` before rebuilding.
+
+
+Source Languages
+================
+
+What source languages are supported?
+------------------------------------
+LLVM currently has full support for C and C++ source languages. These are
+available through both `Clang <http://clang.llvm.org/>`_ and `DragonEgg
+<http://dragonegg.llvm.org/>`_.
+
+The PyPy developers are working on integrating LLVM into the PyPy backend so
+that PyPy language can translate to LLVM.
+
+
+I'd like to write a self-hosting LLVM compiler. How should I interface with the LLVM middle-end optimizers and back-end code generators?
+----------------------------------------------------------------------------------------------------------------------------------------
+Your compiler front-end will communicate with LLVM by creating a module in the
+LLVM intermediate representation (IR) format. Assuming you want to write your
+language's compiler in the language itself (rather than C++), there are 3
+major ways to tackle generating LLVM IR from a front-end:
+
+1. **Call into the LLVM libraries code using your language's FFI (foreign
+   function interface).**
+
+  * *for:* best tracks changes to the LLVM IR, .ll syntax, and .bc format
+
+  * *for:* enables running LLVM optimization passes without a emit/parse
+    overhead
+
+  * *for:* adapts well to a JIT context
+
+  * *against:* lots of ugly glue code to write
+
+2. **Emit LLVM assembly from your compiler's native language.**
+
+  * *for:* very straightforward to get started
+
+  * *against:* the .ll parser is slower than the bitcode reader when
+    interfacing to the middle end
+
+  * *against:* it may be harder to track changes to the IR
+
+3. **Emit LLVM bitcode from your compiler's native language.**
+
+  * *for:* can use the more-efficient bitcode reader when interfacing to the
+    middle end
+
+  * *against:* you'll have to re-engineer the LLVM IR object model and bitcode
+    writer in your language
+
+  * *against:* it may be harder to track changes to the IR
+
+If you go with the first option, the C bindings in include/llvm-c should help
+a lot, since most languages have strong support for interfacing with C. The
+most common hurdle with calling C from managed code is interfacing with the
+garbage collector. The C interface was designed to require very little memory
+management, and so is straightforward in this regard.
+
+What support is there for a higher level source language constructs for building a compiler?
+--------------------------------------------------------------------------------------------
+Currently, there isn't much. LLVM supports an intermediate representation
+which is useful for code representation but will not support the high level
+(abstract syntax tree) representation needed by most compilers. There are no
+facilities for lexical nor semantic analysis.
+
+
+I don't understand the ``GetElementPtr`` instruction. Help!
+-----------------------------------------------------------
+See `The Often Misunderstood GEP Instruction <GetElementPtr.html>`_.
+
+
+Using the C and C++ Front Ends
+==============================
+
+Can I compile C or C++ code to platform-independent LLVM bitcode?
+-----------------------------------------------------------------
+No. C and C++ are inherently platform-dependent languages. The most obvious
+example of this is the preprocessor. A very common way that C code is made
+portable is by using the preprocessor to include platform-specific code. In
+practice, information about other platforms is lost after preprocessing, so
+the result is inherently dependent on the platform that the preprocessing was
+targeting.
+
+Another example is ``sizeof``. It's common for ``sizeof(long)`` to vary
+between platforms. In most C front-ends, ``sizeof`` is expanded to a
+constant immediately, thus hard-wiring a platform-specific detail.
+
+Also, since many platforms define their ABIs in terms of C, and since LLVM is
+lower-level than C, front-ends currently must emit platform-specific IR in
+order to have the result conform to the platform ABI.
+
+
+Questions about code generated by the demo page
+===============================================
+
+What is this ``llvm.global_ctors`` and ``_GLOBAL__I_a...`` stuff that happens when I ``#include <iostream>``?
+-------------------------------------------------------------------------------------------------------------
+If you ``#include`` the ``<iostream>`` header into a C++ translation unit,
+the file will probably use the ``std::cin``/``std::cout``/... global objects.
+However, C++ does not guarantee an order of initialization between static
+objects in different translation units, so if a static ctor/dtor in your .cpp
+file used ``std::cout``, for example, the object would not necessarily be
+automatically initialized before your use.
+
+To make ``std::cout`` and friends work correctly in these scenarios, the STL
+that we use declares a static object that gets created in every translation
+unit that includes ``<iostream>``.  This object has a static constructor
+and destructor that initializes and destroys the global iostream objects
+before they could possibly be used in the file.  The code that you see in the
+``.ll`` file corresponds to the constructor and destructor registration code.
+
+If you would like to make it easier to *understand* the LLVM code generated
+by the compiler in the demo page, consider using ``printf()`` instead of
+``iostream``\s to print values.
+
+
+Where did all of my code go??
+-----------------------------
+If you are using the LLVM demo page, you may often wonder what happened to
+all of the code that you typed in.  Remember that the demo script is running
+the code through the LLVM optimizers, so if your code doesn't actually do
+anything useful, it might all be deleted.
+
+To prevent this, make sure that the code is actually needed.  For example, if
+you are computing some expression, return the value from the function instead
+of leaving it in a local variable.  If you really want to constrain the
+optimizer, you can read from and assign to ``volatile`` global variables.
+
+
+What is this "``undef``" thing that shows up in my code?
+--------------------------------------------------------
+``undef`` is the LLVM way of representing a value that is not defined.  You
+can get these if you do not initialize a variable before you use it.  For
+example, the C function:
+
+.. code-block:: c
+
+   int X() { int i; return i; }
+
+Is compiled to "``ret i32 undef``" because "``i``" never has a value specified
+for it.
+
+
+Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into "unreachable"? Why not make the verifier reject it?
+----------------------------------------------------------------------------------------------------------------------------------------------------------
+This is a common problem run into by authors of front-ends that are using
+custom calling conventions: you need to make sure to set the right calling
+convention on both the function and on each call to the function.  For
+example, this code:
+
+.. code-block:: llvm
+
+   define fastcc void @foo() {
+       ret void
+   }
+   define void @bar() {
+       call void @foo()
+       ret void
+   }
+
+Is optimized to:
+
+.. code-block:: llvm
+
+   define fastcc void @foo() {
+       ret void
+   }
+   define void @bar() {
+       unreachable
+   }
+
+... with "``opt -instcombine -simplifycfg``".  This often bites people because
+"all their code disappears".  Setting the calling convention on the caller and
+callee is required for indirect calls to work, so people often ask why not
+make the verifier reject this sort of thing.
+
+The answer is that this code has undefined behavior, but it is not illegal.
+If we made it illegal, then every transformation that could potentially create
+this would have to ensure that it doesn't, and there is valid code that can
+create this sort of construct (in dead code).  The sorts of things that can
+cause this to happen are fairly contrived, but we still need to accept them.
+Here's an example:
+
+.. code-block:: llvm
+
+   define fastcc void @foo() {
+       ret void
+   }
+   define internal void @bar(void()* %FP, i1 %cond) {
+       br i1 %cond, label %T, label %F
+   T:
+       call void %FP()
+       ret void
+   F:
+       call fastcc void %FP()
+       ret void
+   }
+   define void @test() {
+       %X = or i1 false, false
+       call void @bar(void()* @foo, i1 %X)
+       ret void
+   }
+
+In this example, "test" always passes ``@foo``/``false`` into ``bar``, which
+ensures that it is dynamically called with the right calling conv (thus, the
+code is perfectly well defined).  If you run this through the inliner, you
+get this (the explicit "or" is there so that the inliner doesn't dead code
+eliminate a bunch of stuff):
+
+.. code-block:: llvm
+
+   define fastcc void @foo() {
+       ret void
+   }
+   define void @test() {
+       %X = or i1 false, false
+       br i1 %X, label %T.i, label %F.i
+   T.i:
+       call void @foo()
+       br label %bar.exit
+   F.i:
+       call fastcc void @foo()
+       br label %bar.exit
+   bar.exit:
+       ret void
+   }
+
+Here you can see that the inlining pass made an undefined call to ``@foo``
+with the wrong calling convention.  We really don't want to make the inliner
+have to know about this sort of thing, so it needs to be valid code.  In this
+case, dead code elimination can trivially remove the undefined code.  However,
+if ``%X`` was an input argument to ``@test``, the inliner would produce this:
+
+.. code-block:: llvm
+
+   define fastcc void @foo() {
+       ret void
+   }
+
+   define void @test(i1 %X) {
+       br i1 %X, label %T.i, label %F.i
+   T.i:
+       call void @foo()
+       br label %bar.exit
+   F.i:
+       call fastcc void @foo()
+       br label %bar.exit
+   bar.exit:
+       ret void
+   }
+
+The interesting thing about this is that ``%X`` *must* be false for the
+code to be well-defined, but no amount of dead code elimination will be able
+to delete the broken call as unreachable.  However, since
+``instcombine``/``simplifycfg`` turns the undefined call into unreachable, we
+end up with a branch on a condition that goes to unreachable: a branch to
+unreachable can never happen, so "``-inline -instcombine -simplifycfg``" is
+able to produce:
+
+.. code-block:: llvm
+
+   define fastcc void @foo() {
+      ret void
+   }
+   define void @test(i1 %X) {
+   F.i:
+      call fastcc void @foo()
+      ret void
+   }

Modified: llvm/trunk/docs/userguides.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/userguides.rst?rev=158677&r1=158676&r2=158677&view=diff
==============================================================================
--- llvm/trunk/docs/userguides.rst (original)
+++ llvm/trunk/docs/userguides.rst Mon Jun 18 15:21:38 2012
@@ -7,6 +7,7 @@
    :hidden:
 
    CommandGuide/index
+   FAQ
 
 \
 
@@ -45,7 +46,7 @@
 
     A list of optimizations and analyses implemented in LLVM.
     
- * `Frequently Asked Questions <FAQ.html>`_
+ * :ref:`faq`
 
     A list of common questions and problems and their solutions.
     





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