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161 lines
5.9 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>LLVM gold plugin</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">LLVM gold plugin</div>
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#build">How to build it</a></li>
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<li><a href="#usage">Usage</a>
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<ul>
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<li><a href="#example1">Example of link time optimization</a></li>
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</ul></li>
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<li><a href="#licensing">Licensing</a></li>
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</ol>
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<div class="doc_author">Written by Nick Lewycky</div>
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<!--=========================================================================-->
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<div class="doc_section"><a name="introduction">Introduction</a></div>
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<!--=========================================================================-->
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<div class="doc_text">
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<p>Building with link time optimization requires cooperation from the
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system linker. LTO support on Linux systems requires that you use
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the <a href="http://sourceware.org/binutils">gold linker</a> which supports
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LTO via plugins. This is the same system used by the upcoming
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<a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a>
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project.</p>
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<p>The LLVM gold plugin implements the
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<a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a>
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on top of
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<a href="http://llvm.org/docs/LinkTimeOptimization.html#lto">libLTO</a>.
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The same plugin can also be used by other tools such as <tt>ar</tt> and
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<tt>nm</tt>.
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</div>
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<!--=========================================================================-->
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<div class="doc_section"><a name="build">How to build it</a></div>
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<!--=========================================================================-->
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<div class="doc_text">
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<p>You need to build gold with plugin support and build the LLVMgold
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plugin.</p>
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<ul>
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<li>Build gold with plugin support:
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<pre class="doc_code">
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mkdir binutils
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cd binutils
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cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
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<em>{enter "anoncvs" as the password}</em>
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cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co src
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mkdir build
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cd build
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../src/configure --enable-gold --enable-plugins
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make all-gold
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</pre>
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That should leave you with binutils/build/gold/ld-new which supports the
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<tt>-plugin</tt> option.
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<li>Build the LLVMgold plugin: Configure LLVM with
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<tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
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<tt>make</tt>.
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</ul>
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</div>
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<!--=========================================================================-->
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<div class="doc_section"><a name="usage">Usage</a></div>
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<!--=========================================================================-->
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<div class="doc_text">
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<p>The linker takes a <tt>-plugin</tt> option that points to the path of
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the plugin <tt>.so</tt> file. To find out what link command <tt>gcc</tt>
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would run in a given situation, run <tt>gcc -v <em>[...]</em></tt> and look
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for the line where it runs <tt>collect2</tt>. Replace that with
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<tt>ld-new -plugin /path/to/LLVMgold.so</tt> to test it out. Once you're
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ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
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then replace it with <tt>ld-new</tt>.</p>
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<p>You can produce bitcode files from <tt>llvm-gcc</tt> using
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<tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
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synonymous with <tt>-O3 -flto</tt>.</p>
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<p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
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for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
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passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
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linker, which is why you need gold to be the installed system linker in your
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path.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="example1">Example of link time optimization</a>
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</div>
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<div class="doc_text">
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<p>The following example shows a worked example of the gold plugin mixing
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LLVM bitcode and native code.
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<pre class="doc_code">
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--- a.c ---
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#include <stdio.h>
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extern void foo1(void);
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extern void foo4(void);
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void foo2(void) {
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printf("Foo2\n");
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}
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void foo3(void) {
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foo4();
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}
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int main(void) {
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foo1();
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}
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--- b.c ---
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#include <stdio.h>
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extern void foo2(void);
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void foo1(void) {
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foo2();
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}
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void foo4(void) {
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printf("Foo4");
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}
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--- command lines ---
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$ llvm-gcc -flto a.c -c -o a.o # <-- a.o is LLVM bitcode file
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$ llvm-gcc b.c -c -o b.o # <-- b.o is native object file
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$ llvm-gcc -use-gold-plugin a.o b.o -o main # <-- link with LLVMgold plugin
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</pre>
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<p>Gold informs the plugin that foo3 is never referenced outside the IR,
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leading LLVM to delete that function. However, unlike in the
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<a href="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
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example</a> gold does not currently eliminate foo4.</p>
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</div>
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<!--=========================================================================-->
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<div class="doc_section"><a name="licensing">Licensing</a></div>
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<!--=========================================================================-->
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<div class="doc_text">
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<p>Gold is licensed under the GPLv3. LLVMgold uses the interface file
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<tt>plugin-api.h</tt> from gold which means that the resulting LLVMgold.so
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binary is also GPLv3. This can still be used to link non-GPLv3 programs just
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as much as gold could without the plugin.</p>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
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<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
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</address>
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</body>
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</html>
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