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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Minor alignment change may cause big performance variation in MultiSource/Benchmarks/Ptrdist/ks"
href="https://llvm.org/bugs/show_bug.cgi?id=24570">24570</a>
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<th>Summary</th>
<td>Minor alignment change may cause big performance variation in MultiSource/Benchmarks/Ptrdist/ks
</td>
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<th>Product</th>
<td>Test Suite
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Programs Tests
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>wmi@google.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>When I patched the change in <a href="http://reviews.llvm.org/D12107">http://reviews.llvm.org/D12107</a> to r243652, I saw
-15% perf regression in MultiSource/Benchmarks/Ptrdist/ks.
Analysis using perf showed the regression was caused by a minor alignment
change. The alignment change prevented a cmp and jmp pair inside the kernel
loop of the test from being macro-fused, which increased uops-retired events a
lot.
With the patch
4012b0: 48 39 41 08 cmp %rax,0x8(%rcx)
4012b4: 75 04 jne 4012ba <FindMaxGpAndSwap+0xda>
4012b6: f3 0f 58 e5 addss %xmm5,%xmm4
4012ba: 48 8b 09 mov (%rcx),%rcx
4012bd: 48 85 c9 test %rcx,%rcx
4012c0: 75 ee jne 4012b0 <FindMaxGpAndSwap+0xd0>
Without the patch
4012a0: 48 39 41 08 cmp %rax,0x8(%rcx)
4012a4: 75 04 jne 4012aa <FindMaxGpAndSwap+0xda>
4012a6: f3 0f 58 e5 addss %xmm5,%xmm4
4012aa: 48 8b 09 mov (%rcx),%rcx
4012ad: 48 85 c9 test %rcx,%rcx
4012b0: 75 ee jne 4012a0 <FindMaxGpAndSwap+0xd0>
With the patch, the test and jne pair in the kernel loop crossed 32 bytes
boundary, so they cannot be macro-fused.
The bug was filed to track a perf flaky test on one hand, and to record a
microarchitecture dependent perf tuning opportunity on another hand.</pre>
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