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Fix up the formating and change llvm-gcc to clang.

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Note that this example doesn't work anymore!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139999 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2011-09-18 12:51:05 +00:00
parent ea55c83e7f
commit 88816ec963
1 changed files with 32 additions and 25 deletions
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57
docs/LinkTimeOptimization.html
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@ -79,7 +79,7 @@ conservative escape analysis.
<p>The following example illustrates the advantages of LTO's integrated
approach and clean interface. This example requires a system linker which
supports LTO through the interface described in this document. Here,
llvm-gcc transparently invokes system linker. </p>
clang transparently invokes system linker. </p>
<ul>
<li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode form.
<li> Input source file <tt>main.c</tt> is compiled into native object code.
@ -89,27 +89,29 @@ conservative escape analysis.
extern int foo1(void);
extern void foo2(void);
extern void foo4(void);
--- a.c ---
#include "a.h"
static signed int i = 0;
void foo2(void) {
i = -1;
i = -1;
}
static int foo3() {
foo4();
return 10;
foo4();
return 10;
}
int foo1(void) {
int data = 0;
int data = 0;
if (i &lt; 0) { data = foo3(); }
if (i &lt; 0)
data = foo3();
data = data + 42;
return data;
data = data + 42;
return data;
}
--- main.c ---
@ -117,30 +119,35 @@ return data;
#include "a.h"
void foo4(void) {
printf ("Hi\n");
printf("Hi\n");
}
int main() {
return foo1();
return foo1();
}
--- command lines ---
$ llvm-gcc --emit-llvm -c a.c -o a.o # &lt;-- a.o is LLVM bitcode file
$ llvm-gcc -c main.c -o main.o # &lt;-- main.o is native object file
$ llvm-gcc a.o main.o -o main # &lt;-- standard link command without any modifications
$ clang -emit-llvm -c a.c -o a.o # &lt;-- a.o is LLVM bitcode file
$ clang -c main.c -o main.o # &lt;-- main.o is native object file
$ clang a.o main.o -o main # &lt;-- standard link command without any modifications
</pre>
<p>In this example, the linker recognizes that <tt>foo2()</tt> is an
externally visible symbol defined in LLVM bitcode file. The linker completes
its usual symbol resolution
pass and finds that <tt>foo2()</tt> is not used anywhere. This information
is used by the LLVM optimizer and it removes <tt>foo2()</tt>. As soon as
<tt>foo2()</tt> is removed, the optimizer recognizes that condition
<tt>i &lt; 0</tt> is always false, which means <tt>foo3()</tt> is never
used. Hence, the optimizer removes <tt>foo3()</tt>, also. And this in turn,
enables linker to remove <tt>foo4()</tt>. This example illustrates the
advantage of tight integration with the linker. Here, the optimizer can not
remove <tt>foo3()</tt> without the linker's input.
</p>
<ul>
<li>In this example, the linker recognizes that <tt>foo2()</tt> is an
externally visible symbol defined in LLVM bitcode file. The linker
completes its usual symbol resolution pass and finds that <tt>foo2()</tt>
is not used anywhere. This information is used by the LLVM optimizer and
it removes <tt>foo2()</tt>.</li>
<li>As soon as <tt>foo2()</tt> is removed, the optimizer recognizes that condition
<tt>i &lt; 0</tt> is always false, which means <tt>foo3()</tt> is never
used. Hence, the optimizer also removes <tt>foo3()</tt>.</li>
<li>And this in turn, enables linker to remove <tt>foo4()</tt>.</li>
</ul>
<p>This example illustrates the advantage of tight integration with the
linker. Here, the optimizer can not remove <tt>foo3()</tt> without the
linker's input.</p>
</div>
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