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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [CodeGen] suboptimal code for ffs()"
href="https://llvm.org/bugs/show_bug.cgi?id=31399">31399</a>
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<th>Summary</th>
<td>[CodeGen] suboptimal code for ffs()
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<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>Common Code Generator Code
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>davide@freebsd.org
</td>
</tr>
<tr>
<th>CC</th>
<td>filcab@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, simon.f.whittaker@gmail.com
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Given
#include <strings.h>
int f(int x) {
return ffs(x) + 1;
}
compiled with -O3, clang 3.9.0 generates
f(int): # @f(int)
bsfl %edi, %ecx
addl $2, %ecx
testl %edi, %edi
movl $1, %eax
cmovnel %ecx, %eax
retq
while gcc 6.2 generates:
f(int):
bsf eax, edi
mov edx, -1
cmove eax, edx
add eax, 2
ret
We could save a test instruction. I don't think it matters a lot in practice,
but maybe a fun exercise if somebody wants to take a look (cc:ing Simon W. who
was looking for small codegen bugs to fix).
(Note that we lower ffs() to (x != 0) ? llvm.cttz(x) + 1 : 0 in
SimplifyLibCalls.cpp)</pre>
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</p>
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