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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 - Opportunity to use BTS instruction instead of cmpxchg"
href="https://bugs.llvm.org/show_bug.cgi?id=37974">37974</a>
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</tr>
<tr>
<th>Summary</th>
<td>Opportunity to use BTS instruction instead of cmpxchg
</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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Hello,
Code:
bool foo(int *n, int bit) {
unsigned mask = (1u << bit);
return (__sync_fetch_and_or (n, mask) & mask) != 0;
}
Clang -O3:
foo(int*, int):
mov edx, 1
mov ecx, esi
shl edx, cl
mov eax, dword ptr [rdi]
.LBB0_1:
mov ecx, eax
or ecx, edx
lock cmpxchg dword ptr [rdi], ecx
jne .LBB0_1
test eax, edx
setne al
ret
GCC 7+ generates code using BTS instruction:
foo(int*, int):
lock bts DWORD PTR [rdi], esi
setc al
ret
For more code examples see link:
<a href="https://godbolt.org/g/28nkRu">https://godbolt.org/g/28nkRu</a></pre>
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