minor tweaks

[oota-llvm.git] / docs / GoldPlugin.html

diff --git a/docs/GoldPlugin.html b/docs/GoldPlugin.html
index 2808dfa0b1c6b2b13d425c0e403f13954e35e191..a16b1101ad199a3931f4d81a887ccfece42ec63c 100644 (file)
--- a/docs/GoldPlugin.html
+++ b/docs/GoldPlugin.html
@@ -11,7 +11,10 @@
 <ol>
   <li><a href="#introduction">Introduction</a></li>
   <li><a href="#build">How to build it</a></li>
-  <li><a href="#usage">Usage</a></li>
+  <li><a href="#usage">Usage</a>
+  <ul>
+    <li><a href="#example1">Example of link time optimization</a></li>
+  </ul></li>
   <li><a href="#licensing">Licensing</a></li>
 </ol>
 <div class="doc_author">Written by Nick Lewycky</div>
@@ -20,15 +23,15 @@
 <div class="doc_section"><a name="introduction">Introduction</a></div>
 <!--=========================================================================-->
 <div class="doc_text">
-  <p>Building with link time optimization requires cooperation with the
-system linker. To support LTO on Linux systems, we requires that you use
-<a href="http://sourceware.org/binutils">gold</a> which has support for
+  <p>Building with link time optimization requires cooperation from the
+system linker. LTO support on Linux systems requires that you use
+the <a href="http://sourceware.org/binutils">gold linker</a> which supports
 LTO via plugins. This is the same system used by the upcoming
 <a href="http://gcc.gnu.org/wiki/LinkTimeOptimization">GCC LTO</a>
-support.</p>
-  <p>The LLVMgold plugin implements the gold
-<a href="http://gcc.gnu.org/wiki/whopr/driver">plugin interface</a> on
-top of
+project.</p>
+  <p>The LLVM gold plugin implements the
+<a href="http://gcc.gnu.org/wiki/whopr/driver">gold plugin interface</a>
+on top of
 <a href="http://llvm.org/docs/LinkTimeOptimization.html#lto">libLTO</a>.
 The same plugin can also be used by other tools such as <tt>ar</tt> and
 <tt>nm</tt>.
@@ -53,9 +56,9 @@ cd build
 make all-gold
 </pre>
     That should leave you with binutils/build/gold/ld-new which supports the
--plugin option.
+<tt>-plugin</tt> option.
 
-    <li>Build LLVMgold. Configure LLVM with
+    <li>Build the LLVMgold plugin: Configure LLVM with
     <tt>--with-binutils-include=/path/to/binutils/src/include</tt> and run
     <tt>make</tt>.
 </ul>
@@ -72,13 +75,66 @@ make all-gold
   ready to switch to using gold, backup your existing <tt>/usr/bin/ld</tt>
   then replace it with <tt>ld-new</tt>.</p>
   <p>You can produce bitcode files from <tt>llvm-gcc</tt> using
-  <tt>-emit-llvm</tt> or <tt>-flto</tt> or <tt>-O4</tt> which is equivalent
-  to <tt>-O3 -flto</tt>.</p>
+  <tt>-emit-llvm</tt> or <tt>-flto</tt>, or the <tt>-O4</tt> flag which is
+  synonymous with <tt>-O3 -flto</tt>.</p>
   <p><tt>llvm-gcc</tt> has a <tt>-use-gold-plugin</tt> option which looks
-  for the gold plugin in the same directories as it looks for <tt>cc1</tt>.
-  It will not look for an alternate linker, which is why you need gold to be
-  the installed system linker in your path.</p>
+  for the gold plugin in the same directories as it looks for <tt>cc1</tt> and
+  passes the <tt>-plugin</tt> option to ld. It will not look for an alternate
+  linker, which is why you need gold to be the installed system linker in your
+  path.</p>
 </div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsection">
+  <a name="example1">Example of link time optimization</a>
+</div>
+
+<div class="doc_text">
+  <p>The following example shows a worked example of the gold plugin mixing
+  LLVM bitcode and native code.
+<pre class="doc_code">
+--- a.c ---
+#include &lt;stdio.h&gt;
+
+extern void foo1(void);
+extern void foo4(void);
+
+void foo2(void) {
+  printf("Foo2\n");
+}
+
+void foo3(void) {
+  foo4();
+}
+
+int main(void) {
+  foo1();
+}
+
+--- b.c ---
+#include &lt;stdio.h&gt;
+
+extern void foo2(void);
+
+void foo1(void) {
+  foo2();
+}
+
+void foo4(void) {
+  printf("Foo4");
+}
+
+--- command lines ---
+$ llvm-gcc -flto a.c -c -o a.o              # &lt;-- a.o is LLVM bitcode file
+$ llvm-gcc b.c -c -o b.o                    # &lt;-- b.o is native object file
+$ llvm-gcc -use-gold-plugin a.o b.o -o main # &lt;-- link with LLVMgold plugin
+</pre>
+  <p>Gold informs the plugin that foo3 is never referenced outside the IR,
+  leading LLVM to delete that function. However, unlike in the
+  <a href="http://llvm.org/docs/LinkTimeOptimization.html#example1">libLTO
+  example</a> gold does not currently eliminate foo4.</p>
+</div>
+
 <!--=========================================================================-->
 <div class="doc_section"><a name="licensing">Licensing</a></div>
 <!--=========================================================================-->
@@ -96,7 +152,6 @@ as much as gold could without the plugin.
   src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
   <a href="http://validator.w3.org/check/referer"><img
   src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
-  Written by the 
   <a href="mailto:nicholas@metrix.on.ca">Nick Lewycky</a><br>
   <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
   Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $