forked from OSchip/llvm-project
693 lines
23 KiB
HTML
693 lines
23 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
<html>
|
|
<head>
|
|
<title>LLVM: Frequently Asked Questions</title>
|
|
<style type="text/css">
|
|
@import url("llvm.css");
|
|
.question { font-weight: bold }
|
|
.answer { margin-left: 2em }
|
|
</style>
|
|
</head>
|
|
<body>
|
|
|
|
<div class="doc_title">
|
|
LLVM: Frequently Asked Questions
|
|
</div>
|
|
|
|
<ol>
|
|
<li><a href="#license">License</a>
|
|
<ol>
|
|
<li>Why are the LLVM source code and the front-end distributed under different
|
|
licenses?</li>
|
|
<li>Does the University of Illinois Open Source License really qualify as an
|
|
"open source" license?</li>
|
|
<li>Can I modify LLVM source code and redistribute the modified source?</li>
|
|
<li>Can I modify LLVM source code and redistribute binaries or other tools
|
|
based on it, without redistributing the source?</li>
|
|
</ol></li>
|
|
|
|
<li><a href="#source">Source code</a>
|
|
<ol>
|
|
<li>In what language is LLVM written?</li>
|
|
<li>How portable is the LLVM source code?</li>
|
|
</ol></li>
|
|
|
|
<li><a href="#build">Build Problems</a>
|
|
<ol>
|
|
<li>When I run configure, it finds the wrong C compiler.</li>
|
|
<li>I compile the code, and I get some error about <tt>/localhome</tt>.</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 CVS, and now my build is trying to use a
|
|
file/directory that doesn't exist.</li>
|
|
<li>I've modified a Makefile in my source tree, but my build tree keeps using
|
|
the old version. What do I do?</li>
|
|
<li>I've upgraded to a new version of LLVM, and I get strange build
|
|
errors.</li>
|
|
<li>I've built LLVM and am testing it, but the tests freeze.</li>
|
|
<li>Why do test results differ when I perform different types of builds?</li>
|
|
<li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
|
|
<li>When I use the test suite, all of the C Backend tests fail. What is
|
|
wrong?</li>
|
|
<li>After CVS 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="#langhlsupp">What support is there for higher level source
|
|
language constructs for building a compiler?</a></li>
|
|
<li><a href="GetElementPtr.html">I don't understand the GetElementPtr
|
|
instruction. Help!</a></li>
|
|
</ol>
|
|
|
|
<li><a href="#cfe">Using the GCC Front End</a>
|
|
<ol>
|
|
<li>
|
|
When I compile software that uses a configure script, the configure script
|
|
thinks my system has all of the header files and libraries it is testing
|
|
for. How do I get configure to work correctly?
|
|
</li>
|
|
|
|
<li>
|
|
When I compile code using the LLVM GCC front end, it complains that it
|
|
cannot find libcrtend.a.
|
|
</li>
|
|
|
|
<li>
|
|
How can I disable all optimizations when compiling code using the LLVM GCC front end?
|
|
</li>
|
|
|
|
<li><a href="#translatec++">Can I use LLVM to convert C++ code to C code?</a></li>
|
|
|
|
</ol>
|
|
</li>
|
|
|
|
<li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
|
|
<ol>
|
|
<li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
|
|
<tt>main()</tt>?</a></li>
|
|
<li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
|
|
<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
|
|
#include <iostream>?</a></li>
|
|
<li><a href="#codedce">Where did all of my code go??</a></li>
|
|
<li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
|
|
</ol>
|
|
</li>
|
|
</ol>
|
|
|
|
<div class="doc_author">
|
|
<p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
|
|
</div>
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="license">License</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="question">
|
|
<p>Why are the LLVM source code and the front-end distributed under different
|
|
licenses?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
|
|
Our aim is to distribute LLVM source code under a <em>much less restrictive</em>
|
|
license, in particular one that does not compel users who distribute tools based
|
|
on modifying the source to redistribute the modified source code as well.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>Does the University of Illinois Open Source License really qualify as an
|
|
"open source" license?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Yes, the license is <a
|
|
href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by the Open
|
|
Source Initiative (OSI).</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>Can I modify LLVM source code and redistribute the modified source?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Yes. The modified source distribution must retain the copyright notice and
|
|
follow the three bulletted conditions listed in the <a
|
|
href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>Can I modify LLVM source code and redistribute binaries or other tools based
|
|
on it, without redistributing the source?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Yes, this is why we distribute LLVM under a less restrictive license than
|
|
GPL, as explained in the first question above.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="source">Source Code</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="question">
|
|
<p>In what language is LLVM written?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>All of the LLVM tools and libraries are written in C++ with extensive use of
|
|
the STL.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>How portable is the LLVM source code?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>The LLVM source code should be portable to most modern UNIX-like operating
|
|
systems. Most of the code is written in standard C++ with operating system
|
|
services abstracted to a support library. The tools required to build and test
|
|
LLVM have been ported to a plethora of platforms.</p>
|
|
|
|
<p>Some porting problems may exist in the following areas:</p>
|
|
|
|
<ul>
|
|
|
|
<li>The GCC front end code is not as portable as the LLVM suite, so it may not
|
|
compile as well on unsupported platforms.</li>
|
|
|
|
<li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
|
|
Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
|
|
require more effort.</li>
|
|
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="build">Build Problems</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="question">
|
|
<p>When I run configure, it finds the wrong C compiler.</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
|
|
<p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
|
|
<tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and <tt>CXX</tt>
|
|
for the C and C++ compiler, respectively.</p>
|
|
|
|
<p>If <tt>configure</tt> finds the wrong compiler, either adjust your
|
|
<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
|
|
explicitly.</p>
|
|
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>I compile the code, and I get some error about <tt>/localhome</tt>.</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
|
|
<p>There are several possible causes for this. The first is that you didn't set
|
|
a pathname properly when using <tt>configure</tt>, and it defaulted to a
|
|
pathname that we use on our research machines.</p>
|
|
|
|
<p>Another possibility is that we hardcoded a path in our Makefiles. If you see
|
|
this, please email the LLVM bug mailing list with the name of the offending
|
|
Makefile and a description of what is wrong with it.</p>
|
|
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>The <tt>configure</tt> script finds the right C compiler, but it uses the
|
|
LLVM linker from a previous build. What do I do?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
|
|
if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
|
|
it:</p>
|
|
|
|
<ol>
|
|
|
|
<li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
|
|
program appears first in the <tt>PATH</tt>. This may work, but may not be
|
|
convenient when you want them <i>first</i> in your path for other
|
|
work.</p></li>
|
|
|
|
<li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
|
|
correct. In a Borne compatible shell, the syntax would be:</p>
|
|
|
|
<p><tt>PATH=[the path without the bad program] ./configure ...</tt></p>
|
|
|
|
<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 CVS, 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>
|
|
|
|
<p><tt>./config.status <relative path to Makefile></tt><p>
|
|
|
|
<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>
|
|
|
|
<p><tt>gmake ENABLE_PROFILING=1</tt>
|
|
|
|
<p>...then you must run the tests with the following commands:</p>
|
|
|
|
<p><tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt></p>
|
|
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>Why do test results differ when I perform different types of builds?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
|
|
<p>The LLVM test suite is dependent upon several features of the LLVM tools and
|
|
libraries.</p>
|
|
|
|
<p>First, the debugging assertions in code are not enabled in optimized or
|
|
profiling builds. Hence, tests that used to fail may pass.</p>
|
|
|
|
<p>Second, some tests may rely upon debugging options or behavior that is only
|
|
available in the debug build. These tests will fail in an optimized or profile
|
|
build.</p>
|
|
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>This is <a href="http://gcc.gnu.org/PR?13392">a bug in GCC</a>, and
|
|
affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>If the error is of the form:</p>
|
|
|
|
<div class="doc_code">
|
|
<tt>
|
|
gmake[2]: *** No rule to make target `/path/to/somefile', needed by
|
|
`/path/to/another/file.d'.<br>
|
|
Stop.
|
|
</tt>
|
|
</div>
|
|
|
|
<p>This may occur anytime files are moved within the CVS repository or removed
|
|
entirely. In this case, the best solution is to erase all <tt>.d</tt> files,
|
|
which list dependencies for source files, and rebuild:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
% cd $LLVM_OBJ_DIR
|
|
% rm -f `find . -name \*\.d`
|
|
% gmake
|
|
</pre>
|
|
</div>
|
|
|
|
<p>In other cases, it may be necessary to run <tt>make clean</tt> before
|
|
rebuilding.</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="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>llvm-java</tt> CVS repository. There is no documentation on this yet so
|
|
you'll need to download the code, compile it, and try it.</p>
|
|
<p>In the <tt>examples/BFtoLLVM</tt> directory is a translator for the
|
|
BrainF*** language (2002 Language Specification).</p>
|
|
<p>In the <tt>projects/Stacker</tt> directory is a compiler and runtime
|
|
library for the Stacker language, a "toy" language loosely based on Forth.</p>
|
|
<p>The PyPy developers are working on integrating LLVM into the PyPy backend
|
|
so that PyPy language can translate to LLVM.</p>
|
|
</div>
|
|
<div class="question"><a name="langhlsupp">
|
|
<p>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"><a name="langhlsupp">
|
|
<p>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>.</li>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="cfe">Using the GCC Front End</a>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>
|
|
When I compile software that uses a configure script, the configure script
|
|
thinks my system has all of the header files and libraries it is testing for.
|
|
How do I get configure to work correctly?
|
|
</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>
|
|
The configure script is getting things wrong because the LLVM linker allows
|
|
symbols to be undefined at link time (so that they can be resolved during JIT
|
|
or translation to the C back end). That is why configure thinks your system
|
|
"has everything."
|
|
</p>
|
|
<p>
|
|
To work around this, perform the following steps:
|
|
</p>
|
|
<ol>
|
|
<li>Make sure the CC and CXX environment variables contains the full path to
|
|
the LLVM GCC front end.</li>
|
|
|
|
<li>Make sure that the regular C compiler is first in your PATH. </li>
|
|
|
|
<li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
|
|
</ol>
|
|
|
|
<p>
|
|
This will allow the <tt>llvm-ld</tt> linker to create a native code executable
|
|
instead of shell script that runs the JIT. Creating native code requires
|
|
standard linkage, which in turn will allow the configure script to find out if
|
|
code is not linking on your system because the feature isn't available on your
|
|
system.</p>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>
|
|
When I compile code using the LLVM GCC front end, it complains that it cannot
|
|
find libcrtend.a.
|
|
</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>
|
|
The only way this can happen is if you haven't installed the runtime library. To
|
|
correct this, do:</p>
|
|
<pre>
|
|
% cd llvm/runtime
|
|
% make clean ; make install-bytecode
|
|
</pre>
|
|
</div>
|
|
|
|
<div class="question">
|
|
<p>
|
|
How can I disable all optimizations when compiling code using the LLVM GCC front end?
|
|
</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>
|
|
Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
|
|
optimizations done at the llvm level, leaving you with the truly horrible
|
|
code that you desire.
|
|
</p>
|
|
</div>
|
|
|
|
|
|
<div class="question">
|
|
<p>
|
|
<a name="translatec++">Can I use LLVM to convert C++ code to C code?</a>
|
|
</p>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
<p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
|
|
Note that the generated C code will be very low level (all loops are lowered
|
|
to gotos, etc) and not very pretty (comments are stripped, original source
|
|
formatting is totally lost, variables are renamed, expressions are regrouped),
|
|
so this may not be what you're looking for. However, this is a good way to add
|
|
C++ support for a processor that does not otherwise have a C++ compiler.
|
|
</p>
|
|
|
|
<p>Use commands like this:</p>
|
|
|
|
<ol>
|
|
<li><p>Compile your program as normal with llvm-g++:</p></li>
|
|
|
|
<div class="doc_code">$ llvm-g++ x.cpp -o program</div>
|
|
|
|
<p>or:</p>
|
|
|
|
<div class="doc_code">
|
|
llvm-g++ a.cpp -c<br>
|
|
llvm-g++ b.cpp -c<br>
|
|
llvm-g++ a.o b.o -o program
|
|
</div>
|
|
|
|
<p>With llvm-gcc3, this will generate program and program.bc. The .bc file is
|
|
the LLVM version of the program all linked together.</p>
|
|
|
|
<li><p>Convert the LLVM code to C code, using the LLC tool with the C
|
|
backend:</p></li>
|
|
|
|
<div class="doc_code">$ llc -march=c program.bc -o program.c</div>
|
|
|
|
<li><p>Finally, compile the c file:</p></li>
|
|
|
|
<div class="doc_code">$ cc x.c</div>
|
|
|
|
</ol>
|
|
|
|
<p>Note that, by default, the C backend does not support exception handling.
|
|
If you want/need it for a certain program, you can enable it by passing
|
|
"-enable-correct-eh-support" to the llc program. The resultant code will
|
|
use setjmp/longjmp to implement exception support that is correct but
|
|
relatively slow.
|
|
</p>
|
|
|
|
<p>Also note: this specific sequence of commands won't work if you use a
|
|
function defined in the C++ runtime library (or any other C++ library). To
|
|
access an external C++ library, you must manually
|
|
compile libstdc++ to LLVM bytecode, statically link it into your program, then
|
|
use the commands above to convert the whole result into C code. Alternatively,
|
|
you can compile the libraries and your application into two different chunks
|
|
of C code and link them.</p>
|
|
|
|
</div>
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="cfe_code">Questions about code generated by the GCC front-end</a>
|
|
</div>
|
|
|
|
<div class="question"><p>
|
|
<a name="__main"></a>
|
|
What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
|
|
</p></div>
|
|
|
|
<div class="answer">
|
|
<p>
|
|
The <tt>__main</tt> call is inserted by the C/C++ compiler in order to guarantee
|
|
that static constructors and destructors are called when the program starts up
|
|
and shuts down. In C, you can create static constructors and destructors by
|
|
using GCC extensions, and in C++ you can do so by creating a global variable
|
|
whose class has a ctor or dtor.
|
|
</p>
|
|
|
|
<p>
|
|
The actual implementation of <tt>__main</tt> lives in the
|
|
<tt>llvm/runtime/GCCLibraries/crtend/</tt> directory in the source-base, and is
|
|
linked in automatically when you link the program.
|
|
</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>
|
|
</div>
|
|
|
|
<div class="answer">
|
|
|
|
<p>If you #include the <iostream> header into a C++ translation unit, the
|
|
file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
|
|
objects. However, C++ does not guarantee an order of initialization between
|
|
static objects in different translation units, so if a static ctor/dtor in your
|
|
.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
|
|
be automatically initialized before your use.</p>
|
|
|
|
<p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
|
|
STL that we use declares a static object that gets created in every translation
|
|
unit that includes <tt><iostream></tt>. This object has a static
|
|
constructor and destructor that initializes and destroys the global iostream
|
|
objects before they could possibly be used in the file. The code that you see
|
|
in the .ll file corresponds to the constructor and destructor registration code.
|
|
</p>
|
|
|
|
<p>If you would like to make it easier to <b>understand</b> the LLVM code
|
|
generated by the compiler in the demo page, consider using <tt>printf()</tt>
|
|
instead of <tt>iostream</tt>s to print values.</p>
|
|
|
|
</div>
|
|
|
|
<!--=========================================================================-->
|
|
|
|
<div class="question"><p>
|
|
<a name="codedce"></a>
|
|
Where did all of my code go??
|
|
</p></div>
|
|
|
|
<div class="answer">
|
|
<p>
|
|
If you are using the LLVM demo page, you may often wonder what happened to all
|
|
of the code that you typed in. Remember that the demo script is running the
|
|
code through the LLVM optimizers, so if your code doesn't actually do anything
|
|
useful, it might all be deleted.
|
|
</p>
|
|
|
|
<p>
|
|
To prevent this, make sure that the code is actually needed. For example, if
|
|
you are computing some expression, return the value from the function instead of
|
|
leaving it in a local variable. If you really want to constrain the optimizer,
|
|
you can read from and assign to <tt>volatile</tt> global variables.
|
|
</p>
|
|
</div>
|
|
|
|
<!--=========================================================================-->
|
|
|
|
<div class="question"><p>
|
|
<a name="undef"></a>
|
|
<p>What is this "<tt>undef</tt>" thing that shows up in my code?
|
|
</p></div>
|
|
|
|
<div class="answer">
|
|
<p>
|
|
<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
|
|
a value that is not defined. You can get these if you do not initialize a
|
|
variable before you use it. For example, the C function:</p>
|
|
|
|
<div class="doc_code">
|
|
<tt>int X() { int i; return i; }</tt>
|
|
</div>
|
|
|
|
<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
|
|
specified for it.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|