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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - extra movz [x86, Intel, partial register update]"
href="http://llvm.org/bugs/show_bug.cgi?id=22532">22532</a>
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<th>Summary</th>
<td>extra movz [x86, Intel, partial register update]
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<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>
</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>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>This is related to the discussion in <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM not using cmpb for unsigned char comparison"
href="show_bug.cgi?id=22473">bug 22473</a> and may be the same issue as <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - extra zeroing move operation executed"
href="show_bug.cgi?id=17113">bug
17113</a> but with a simpler test case:
unsigned char f(unsigned char a, unsigned char b) {
return a == b;
}
Or as IR:
define zeroext i8 @g(i8 zeroext %a, i8 zeroext %b) {
%cmp = icmp eq i8 %a, %b
%conv = zext i1 %cmp to i8
ret i8 %conv
}
This becomes:
cmpl %esi, %edi
sete %al
movzbl %al, %eax <--- is this zext good for perf?
retq
There seems to be empirical evidence (see <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM not using cmpb for unsigned char comparison"
href="show_bug.cgi?id=22473">bug 22473</a>) that zexts help
performance on SandyBridge because they help avoid partial register stalls.
This is despite Intel's docs (section 3.5.2.4 of the Optimization Reference
Manual) that suggest this isn't a big deal on SandyBridge or later.
Now, if this is a perf win for some Intel chips, then we're still not getting
it right all the time:
// C99 version: use bool return instead of unsigned char
#include <stdbool.h>
bool g(unsigned char a, unsigned char b) {
return a == b;
}
Or as IR:
define zeroext i1 @g(i8 zeroext %a, i8 zeroext %b) #0 {
%cmp = icmp eq i8 %a, %b
ret i1 %cmp
}
This has no 'movzbl':
cmpl %esi, %edi
sete %al
retq
And regardless of the perf question, we generate the 'movzbl' for f() with -Oz;
that can't be right.</pre>
</div>
</p>
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