[llvm-commits] [llvm] r68528 - /llvm/trunk/docs/FAQ.html
Bill Wendling
isanbard at gmail.com
Tue Apr 7 11:40:56 PDT 2009
Author: void
Date: Tue Apr 7 13:40:56 2009
New Revision: 68528
URL: http://llvm.org/viewvc/llvm-project?rev=68528&view=rev
Log:
Another reformatting. No change in docs.
Modified:
llvm/trunk/docs/FAQ.html
Modified: llvm/trunk/docs/FAQ.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/FAQ.html?rev=68528&r1=68527&r2=68528&view=diff
==============================================================================
--- llvm/trunk/docs/FAQ.html (original)
+++ llvm/trunk/docs/FAQ.html Tue Apr 7 13:40:56 2009
@@ -2,6 +2,7 @@
"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");
@@ -18,78 +19,95 @@
<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>
+ <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>
+ <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>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>
</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>
+ 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>
+ 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>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><a href="#translatecxx">Can I use LLVM to convert C++ code to C code?</a></li>
+ <li>When I compile code using the LLVM GCC front end, it complains that it
+ cannot find libcrtend.a?</li>
- <li><a href="#platformindependent">Can I compile C or C++ code to platform-independent LLVM bitcode?</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>
@@ -98,8 +116,11 @@
<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="#undef">What is this "<tt>undef</tt>" thing that shows up in
+ my code?</a></li>
</ol>
</li>
</ol>
@@ -117,25 +138,26 @@
<div class="question">
<p>Why are the LLVM source code and the front-end distributed under different
-licenses?</p>
+ 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>
+ 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>
+ "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>
+<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">
@@ -144,18 +166,19 @@
<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>
+ 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>
+ 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>
+<p>Yes. This is why we distribute LLVM under a less restrictive license than
+ GPL, as explained in the first question above.</p>
</div>
<!-- *********************************************************************** -->
@@ -170,7 +193,7 @@
<div class="answer">
<p>All of the LLVM tools and libraries are written in C++ with extensive use of
-the STL.</p>
+ the STL.</p>
</div>
<div class="question">
@@ -207,26 +230,25 @@
</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>
+ <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>
+ <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>
+ 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>
+ 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
@@ -245,7 +267,6 @@
to do its work without having to adjust your <tt>PATH</tt>
permanently.</p></li>
</ol>
-
</div>
<div class="question">
@@ -254,38 +275,37 @@
<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>
+ 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>
+<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>
+ 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>
+ 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>
+<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>
-
+ it over.</p>
</div>
<div class="question">
@@ -295,13 +315,12 @@
<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>
+ 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>
-
+ 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">
@@ -309,10 +328,9 @@
</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>
+ (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>
@@ -326,7 +344,6 @@
% cd llvm/test
% gmake ENABLE_PROFILING=1
</pre>
-
</div>
<div class="question">
@@ -334,17 +351,15 @@
</div>
<div class="answer">
-
<p>The LLVM test suite is dependent upon several features of the LLVM tools and
-libraries.</p>
+ 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>
+ 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>
-
+ available in the debug build. These tests will fail in an optimized or
+ profile build.</p>
</div>
<div class="question">
@@ -352,24 +367,26 @@
</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>
+<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>
+<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>
+<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>
+ target".</p>
</div>
<div class="answer">
@@ -381,9 +398,9 @@
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>
+<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
@@ -392,105 +409,121 @@
</pre>
<p>In other cases, it may be necessary to run <tt>make clean</tt> before
-rebuilding.</p>
+ rebuilding.</p>
</div>
<div class="question"><p><a name="llvmc">
-The <tt>llvmc</tt> program gives me errors/doesn't work.</a></p>
+<p>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>
+ 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 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>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>
+<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 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
+<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
Instruction</a>.</p>
</div>
@@ -500,51 +533,44 @@
</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>
+<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>
+<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 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>
+<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>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>
+<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
@@ -553,33 +579,28 @@
</div>
<div class="question">
-<p>
-How can I disable all optimizations when compiling code using the LLVM GCC front end?
-</p>
+<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>
+<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>
+<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>
+ 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>
@@ -590,7 +611,7 @@
% llvm-g++ x.cpp -o program
</pre>
- <p>or:</p>
+ <p>or:</p>
<pre class="doc_code">
% llvm-g++ a.cpp -c
@@ -608,7 +629,7 @@
% llc -march=c program.bc -o program.c
</pre></li>
-<li><p>Finally, compile the C file:</p>
+ <li><p>Finally, compile the C file:</p>
<pre class="doc_code">
% cc x.c
@@ -616,56 +637,51 @@
</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 incompatiblities
-with standard C++ libraries.</p>
-
+<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 incompatiblities 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>
+<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 targetting.</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 hardwaring 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>
-
+<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
+ targetting.</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 hardwaring 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>
<!-- *********************************************************************** -->
@@ -674,77 +690,71 @@
</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>
+<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 #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>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.
+ 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>
-
+ 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="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>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>
+<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="question">
+<p><a name="undef">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>
+<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>
+<p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
+ value specified for it.</p>
</div>
<!-- *********************************************************************** -->
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