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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Missed induction variable optimizations (-m64 -O2 -march=slm)"
href="https://llvm.org/bugs/show_bug.cgi?id=27519">27519</a>
</td>
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<tr>
<th>Summary</th>
<td>Missed induction variable optimizations (-m64 -O2 -march=slm)
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>andrew.senkevich@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=16267" name="attach_16267" title="Small reproducer">attachment 16267</a> <a href="attachment.cgi?id=16267&action=edit" title="Small reproducer">[details]</a></span>
Small reproducer
Hi,
for attached test Clang generates 5% slower code than GCC due to missed
optimizations of induction variable.
Clang assembly for hottest loop:
4005c0: 8b 1f mov (%rdi),%ebx
4005c2: 0f af 1e imul (%rsi),%ebx
4005c5: 83 c2 01 add $0x1,%edx
4005c8: 48 81 c7 24 03 00 00 add $0x324,%rdi
4005cf: 48 83 c6 04 add $0x4,%rsi
4005d3: 01 d9 add %ebx,%ecx
4005d5: 39 d0 cmp %edx,%eax
4005d7: 75 e7 jne 4005c0 <main+0x50>
GCC assembly:
400410: 8b 94 87 60 10 60 00 mov 0x601060(%rdi,%rax,4),%edx
400417: 48 83 c0 01 add $0x1,%rax
40041b: 48 81 c1 24 03 00 00 add $0x324,%rcx
400422: 0f af 91 dc fc ff ff imul -0x324(%rcx),%edx
400429: 01 d6 add %edx,%esi
40042b: 41 39 c0 cmp %eax,%r8d
40042e: 7f e0 jg 400410 <main+0x40>
And the same for outer loop.</pre>
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