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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Slow code (-O2) due to using jne instead of using cmove"
href="https://bugs.llvm.org/show_bug.cgi?id=45690">45690</a>
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<th>Summary</th>
<td>Slow code (-O2) due to using jne instead of using cmove
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>10.0
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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>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>Levo.DeLellis@gmail.com
</td>
</tr>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=23403" name="attach_23403" title="The test cpp file">attachment 23403</a> <a href="attachment.cgi?id=23403&action=edit" title="The test cpp file">[details]</a></span>
The test cpp file
I attached my test file. I compiled with clang -O2 test.cpp -S
I notice branching in my code when I specifically wrote it so it wouldn't
branch. It jumped over a single instruction so I replaced it with a cmove. It
appears to cut this loop time in half
95,97c95
< jne .LBB1_8
< # %bb.7: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
<span class="quote">> cmove %ecx, %edx</span >
105,107c103
< jne .LBB1_10
< # %bb.9: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
<span class="quote">> cmove %ecx, %edx</span >
115,117c111
< jne .LBB1_12
< # %bb.11: # in Loop: Header=BB1_6 Depth=1
< movl %ebx, %edx
---
<span class="quote">> cmove %ebx, %edx</span >
125,127c119
< jne .LBB1_14
< # %bb.13: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
<span class="quote">> cmove %ecx, %edx</span >
135,137c127
< jne .LBB1_16
< # %bb.15: # in Loop: Header=BB1_6 Depth=1
< movl %ebx, %edx
---
<span class="quote">> cmove %ebx, %edx</span >
145,147c135
< jne .LBB1_18
< # %bb.17: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
<span class="quote">> cmove %ecx, %edx</span >
155,157c143
< jne .LBB1_20
< # %bb.19: # in Loop: Header=BB1_6 Depth=1
< movl %ebx, %edx
---
<span class="quote">> cmove %ebx, %edx</span >
165,167c151
< jne .LBB1_22
< # %bb.21: # in Loop: Header=BB1_6 Depth=1
< movl %ecx, %edx
---
<span class="quote">> cmove %ecx, %edx</span ></pre>
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