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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 --- - Using bitwise-or versus logical-or on boolean values produces different assembly at -O3"
href="http://llvm.org/bugs/show_bug.cgi?id=22723">22723</a>
</td>
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<th>Summary</th>
<td>Using bitwise-or versus logical-or on boolean values produces different assembly at -O3
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</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>Linux
</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>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>rtrieu@google.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>Compare:
bool test(bool x, bool y) {
return !x | !y;
}
And:
bool test(bool x, bool y) {
return !x || !y;
}
which only differ in that the first uses a bitwise-or while the second uses a
logical-or. Since the values are bool, both functions should produce the same
result. However, different assembly is produced when compiled with Clang at
-O3.
Bitwise function:
.text
.file "bool1.cc"
.globl _Z4testbb
.align 16, 0x90
.type _Z4testbb,@function
_Z4testbb: # @_Z4testbb
.cfi_startproc
# BB#0: # %entry
movzbl %dil, %ecx
xorl $1, %ecx
movzbl %sil, %eax
xorl $1, %eax
orl %ecx, %eax
# kill: AL<def> AL<kill> EAX<kill>
retq
.Ltmp0:
.size _Z4testbb, .Ltmp0-_Z4testbb
.cfi_endproc
.ident "clang version 3.7.0 (trunk 230662)"
.section ".note.GNU-stack","",@progbits
Logical function:
.text
.file "bool2.cc"
.globl _Z4testbb
.align 16, 0x90
.type _Z4testbb,@function
_Z4testbb: # @_Z4testbb
.cfi_startproc
# BB#0: # %entry
andb %sil, %dil
xorb $1, %dil
movb %dil, %al
retq
.Ltmp0:
.size _Z4testbb, .Ltmp0-_Z4testbb
.cfi_endproc
.ident "clang version 3.7.0 (trunk 230662)"
.section ".note.GNU-stack","",@progbits</pre>
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