mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2025-01-31 20:51:52 +01:00
Another reformatting. No change in docs.
llvm-svn: 68528
This commit is contained in:
Bill Wendling
parent
f57704fbed
commit
fad0a6efa8
640
docs/FAQ.html
640
docs/FAQ.html
@ -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 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>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>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>
|
||||
<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 @@ Source Initiative (OSI).</p>
|
||||
|
||||
<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 @@ GPL, as explained in the first question above.</p>
|
||||
|
||||
<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 @@ LLVM have been ported to a plethora of platforms.</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>
|
||||
<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 @@ it:</p>
|
||||
to do its work without having to adjust your <tt>PATH</tt>
|
||||
permanently.</p></li>
|
||||
</ol>
|
||||
|
||||
</div>
|
||||
|
||||
<div class="question">
|
||||
@ -254,38 +275,37 @@ it:</p>
|
||||
|
||||
<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>
|
||||
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 @@ it over.</p>
|
||||
<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 @@ clean</tt> and then <tt>make</tt> in the directory that fails to build.</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>
|
||||
(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 @@ clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
|
||||
% cd llvm/test
|
||||
% gmake ENABLE_PROFILING=1
|
||||
</pre>
|
||||
|
||||
</div>
|
||||
|
||||
<div class="question">
|
||||
@ -334,17 +351,15 @@ clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
|
||||
</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 @@ build.</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>
|
||||
<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">
|
||||
@ -382,8 +399,8 @@ 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>
|
||||
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 @@ removed entirely. In this case, the best solution is to erase all
|
||||
</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>
|
||||
<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 class="question">
|
||||
<p><a name="langs">What source languages are supported?</a></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>
|
||||
<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="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 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="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>Your compiler front-end will communicate with LLVM by creating a module in
|
||||
the LLVM intermediate representation (IR) format. Assuming you want to write
|
||||
your language's compiler in the language itself (rather than C++), there are
|
||||
3 major ways to tackle generating LLVM IR from a front-end:</p>
|
||||
|
||||
<ul>
|
||||
<li><strong>Call into the LLVM libraries code using your language's FFI
|
||||
(foreign function interface).</strong>
|
||||
|
||||
<ul>
|
||||
<li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc
|
||||
format</li>
|
||||
|
||||
<li><em>for:</em> enables running LLVM optimization passes without a
|
||||
emit/parse overhead</li>
|
||||
|
||||
<li><em>for:</em> adapts well to a JIT context</li>
|
||||
|
||||
<li><em>against:</em> lots of ugly glue code to write</li>
|
||||
</ul></li>
|
||||
|
||||
<li> <strong>Emit LLVM assembly from your compiler's native language.</strong>
|
||||
<ul>
|
||||
<li><em>for:</em> very straightforward to get started</li>
|
||||
|
||||
<li><em>against:</em> the .ll parser is slower than the bitcode reader
|
||||
when interfacing to the middle end</li>
|
||||
|
||||
<li><em>against:</em> you'll have to re-engineer the LLVM IR object model
|
||||
and asm writer in your language</li>
|
||||
|
||||
<li><em>against:</em> it may be harder to track changes to the IR</li>
|
||||
</ul></li>
|
||||
|
||||
<li><strong>Emit LLVM bitcode from your compiler's native language.</strong>
|
||||
|
||||
<ul>
|
||||
<li><em>for:</em> can use the more-efficient bitcode reader when
|
||||
interfacing to the middle end</li>
|
||||
|
||||
<li><em>against:</em> you'll have to re-engineer the LLVM IR object
|
||||
model and bitcode writer in your language</li>
|
||||
|
||||
<li><em>against:</em> it may be harder to track changes to the IR</li>
|
||||
</ul></li>
|
||||
</ul>
|
||||
|
||||
<p>If you go with the first option, the C bindings in include/llvm-c should help
|
||||
a lot, since most languages have strong support for interfacing with C. The
|
||||
most common hurdle with calling C from managed code is interfacing with the
|
||||
garbage collector. The C interface was designed to require very little memory
|
||||
management, and so is straightforward in this regard.</p>
|
||||
</div>
|
||||
|
||||
<div class="question">
|
||||
<p><a name="langhlsupp">What support is there for a higher level source language
|
||||
constructs for building a compiler?</a></p>
|
||||
</div>
|
||||
|
||||
<div class="answer">
|
||||
<p>Currently, there isn't much. LLVM supports an intermediate representation
|
||||
which is useful for code representation but will not support the high level
|
||||
(abstract syntax tree) representation needed by most compilers. There are no
|
||||
facilities for lexical nor semantic analysis. There is, however, a <i>mostly
|
||||
implemented</i> configuration-driven
|
||||
<a href="CompilerDriver.html">compiler driver</a> which simplifies the task
|
||||
of running optimizations, linking, and executable generation.</p>
|
||||
</div>
|
||||
|
||||
<div class="question">
|
||||
<p><a name="getelementptr">I don't understand the GetElementPtr
|
||||
instruction. Help!</a></p>
|
||||
</div>
|
||||
|
||||
<div class="answer">
|
||||
<p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
|
||||
Instruction</a>.</p>
|
||||
</div>
|
||||
|
||||
@ -500,51 +533,44 @@ using <tt>llvm-gcc</tt> instead.</p>
|
||||
</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>
|
||||
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 @@ correct this, do:</p>
|
||||
</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 @@ limitations noted below.<p>
|
||||
% 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 @@ limitations noted below.<p>
|
||||
% 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 @@ limitations noted below.<p>
|
||||
|
||||
</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>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>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>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>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>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 @@ IR in order to have the result conform to the platform ABI.</p>
|
||||
</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>
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user