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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM introduces unneeded shifts in bitfield comparison"
href="http://llvm.org/bugs/show_bug.cgi?id=17956">17956</a>
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<th>Summary</th>
<td>LLVM introduces unneeded shifts in bitfield comparison
</td>
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<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>david.majnemer@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>consider:
struct foo {
int f_x : 1;
int f_y : 1;
};
int foo(struct foo *f) {
return f->f_x == f->f_y;
}
we generate:
movb (%rdi), %al
movb %al, %cl
shlb $6, %cl
sarb $7, %cl
shlb $7, %al
sarb $7, %al
cmpb %cl, %al
sete %al
movzbl %al, %eax
ret
All of those shifts seem superfluous:
movl (%rdi), %ecx
testl %ecx, %ecx
sete %al # set $al to 1 if *f == zero
cmpl $3, %ecx
sete %cl # set $cl to 1 if *f == 3
orb %al, %cl
movzbl %cl, %eax</pre>
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