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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 - Reuse flags from shift instruction -> avoid test instruction"
href="https://bugs.llvm.org/show_bug.cgi?id=43597">43597</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Reuse flags from shift instruction -> avoid test instruction
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.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
</td>
</tr></table>
<p>
<div>
<pre>int foo (int a)
{
int b = 0;
while (a >>= 1)
b++;
return b;
}
foo:
mov eax, -1
.LBB0_1:
sar edi
add eax, 1
test edi, edi
jne .LBB0_1
ret
<a href="https://c9x.me/x86/html/file_module_x86_id_285.html">https://c9x.me/x86/html/file_module_x86_id_285.html</a>
"The SF, ZF, and PF flags are set according to the result. If the count is 0,
the flags are not affected. For a non-zero count, the AF flag is undefined."
So 'test' could be eliminated here.
foo:
mov eax, -1
.LBB0_1:
add eax, 1
sar edi
jne .LBB0_1
ret</pre>
</div>
</p>
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