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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - x86: inefficient code for x == 0 || x == 1"
href="https://bugs.llvm.org/show_bug.cgi?id=32524">32524</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>x86: inefficient code for x == 0 || x == 1
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>4.0
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hans@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Blocks</th>
<td>26299
</td>
</tr></table>
<p>
<div>
<pre>For the example code:
bool f(int x) {
return x == 0 || x == 1;
}
Clang generates (10 bytes):
or edi,0x1
cmp edi,0x1
sete al
ret
GCC and MSVC generate (7 bytes):
cmp edi,0x1
setbe al
ret
At -Os, Clang's code gets even worse, not clear why (14 bytes):
or edi,0x1
mov eax,0x1
cmp edi,eax
sete al
ret
I suppose LLVM notices that the conditions only differ in the least significant
bit, as in the example below where the strategy makes sense:
bool g(int x) {
return x == 4 || x == 5;
}
Clang:
or edi,0x1
cmp edi,0x5
sete al
ret
GCC:
sub edi,0x4
cmp edi,0x1
setbe al
ret
A related case is this one:
bool h(int x) {
return x == -1 || x == 0;
}
Clang (14 bytes):
h(int):
cmp edi,0xffffffff
sete cl
test edi,edi
sete al
or al,cl
ret
GCC (10 bytes):
h(int):
add edi,0x1
cmp edi,0x1
setbe al
ret</pre>
</div>
</p>
<div id="referenced">
<hr style="border: 1px dashed #969696">
<b>Referenced Bugs:</b>
<ul>
<li>
[<a class="bz_bug_link
bz_status_NEW "
title="NEW - [meta][X86] Size optimization opportunities (in particular for 32-bit Chromium on Windows)"
href="https://bugs.llvm.org/show_bug.cgi?id=26299">Bug 26299</a>] [meta][X86] Size optimization opportunities (in particular for 32-bit Chromium on Windows)
</li>
</ul>
</div>
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