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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 --- - [x86] missed chance to turn select (cmov) into logic ops"
href="https://llvm.org/bugs/show_bug.cgi?id=28968">28968</a>
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<th>Summary</th>
<td>[x86] missed chance to turn select (cmov) into logic ops
</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>All
</td>
</tr>
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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>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>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Forking this off of <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [SimplifyCFG] failure to sink common function call below if/else"
href="show_bug.cgi?id=28964">bug 28964</a>.
Notice that:
int goo(int x) {
if (x == 1)
x = 1;
else
x = -1;
return x;
}
Could be rewritten as:
int hoo(int x) {
if (x != 1)
x = -1;
return x;
}
Or as IR:
define i32 @hoo(i32 %x) {
%cmp = icmp ne i32 %x, 1
%sel = select i1 %cmp, i32 -1, i32 1
ret i32 %sel
}
$ ./llc -o - cmpsel.ll
cmpl $1, %edi
movl $-1, %eax
cmovel %edi, %eax
retq
According to godbolt ( <a href="https://godbolt.org/g/TSTeIo">https://godbolt.org/g/TSTeIo</a> ), gcc learned to avoid a
cmov on this at v6.1:
foo(int):
xorl %eax, %eax
cmpl $1, %edi
setne %al
negl %eax
orl $1, %eax
ret
Note that gcc does this at -O2 but not -O1 or -Os.
Filing this as an x86 bug for now, but it might be useful for other targets
too.</pre>
</div>
</p>
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