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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - suboptimal atomic_fetch_sub and atomic_fetch_add"
href="https://bugs.llvm.org/show_bug.cgi?id=43064">43064</a>
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<th>Summary</th>
<td>suboptimal atomic_fetch_sub and atomic_fetch_add
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>8.0
</td>
</tr>
<tr>
<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>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>nruslan_devel@yahoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>I have noticed that if I write code:
#include <stdatomic.h>
int func(_Atomic(int) *a)
{
return (atomic_fetch_sub(a, 1) - 1 == 0);
}
clang/llvm generates optimized code (-O2):
func: # @func
.cfi_startproc
# %bb.0:
xorl %eax, %eax
lock subl $1, (%rdi)
sete %al
retq
But when I change the condition to <= 0, it does not work. Correct me if I am
wrong, but, I think, it should still be able to use sub:
#include <stdatomic.h>
int func(_Atomic(int) *a)
{
return (atomic_fetch_sub(a, 1) - 1 <= 0);
}
func: # @func
.cfi_startproc
# %bb.0:
movl $-1, %ecx
lock xaddl %ecx, (%rdi)
xorl %eax, %eax
cmpl $2, %ecx
setl %al
retq
Seems like the same problem exists for atomic_fetch_add as well.</pre>
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