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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Performance regression with LTO after gold plugin change in r217458"
href="http://llvm.org/bugs/show_bug.cgi?id=20974">20974</a>
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<tr>
<th>Summary</th>
<td>Performance regression with LTO after gold plugin change in r217458
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>gold-plugin
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>avolkov.intel@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, rafael.espindola@gmail.com, zinovy.nis@gmail.com
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>After change in r217458 in gold plugin there is a performance regression with
LTO due to symbol do not internalized.
$ cat a.c
extern int x;
void init_x()
{
x = 7;
}
$ cat b.c
#include <stdlib.h>
void init_x();
int x;
int main()
{
x = 0;
init_x();
return rand() % x;
}
On x86 Sandy Bridge processor running
clang -Ofast -flto b.c a.c
Before change here we have optimized remainder calculation:
00000000004005b0 <main>:
4005b0: 50 push %rax
4005b1: e8 da fe ff ff callq 400490 <rand@plt>
4005b6: 48 98 cltq
4005b8: 48 69 c8 93 24 49 92 imul $0xffffffff92492493,%rax,%rcx
4005bf: 48 c1 e9 20 shr $0x20,%rcx
4005c3: 89 c2 mov %eax,%edx
4005c5: 01 ca add %ecx,%edx
4005c7: 89 d1 mov %edx,%ecx
4005c9: c1 e9 1f shr $0x1f,%ecx
4005cc: c1 fa 02 sar $0x2,%edx
4005cf: 01 ca add %ecx,%edx
4005d1: 6b ca 07 imul $0x7,%edx,%ecx
4005d4: 29 c8 sub %ecx,%eax
4005d6: 5a pop %rdx
4005d7: c3 retq
After change remainder calculated through div with 2x drop in performance:
00000000004005b0 <main>:
4005b0: 50 push %rax
4005b1: c7 05 c1 13 00 00 07 movl $0x7,0x13c1(%rip) #
40197c <x>
4005b8: 00 00 00
4005bb: e8 d0 fe ff ff callq 400490 <rand@plt>
4005c0: 99 cltd
4005c1: f7 3d b5 13 00 00 idivl 0x13b5(%rip) # 40197c <x>
4005c7: 89 d0 mov %edx,%eax
4005c9: 5a pop %rdx
4005ca: c3 retq</pre>
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