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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization for (n << 1) ? X : Y"
href="https://bugs.llvm.org/show_bug.cgi?id=42696">42696</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimization for (n << 1) ? X : Y
</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>
<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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>int test(int n)
{
return (2*n) ? n << 1 : 0;
}
int test2(int n)
{
return (n << 1) ? 2*n : 0;
}
Clang -O3 trunk:
test(int): # @test(int)
lea eax, [rdi + rdi]
ret
test2(int): # @test2(int)
lea eax, [rdi + rdi]
mov ecx, edi
and ecx, 2147483647
test ecx, ecx
cmove eax, ecx
ret
GCC -O3 trunk:
test(int):
lea eax, [rdi+rdi]
ret
test2(int):
lea eax, [rdi+rdi]
ret
Also, GCC's add should be better than TEST with imm?
int test2(int n)
{
return (n << 1) ? 1 : 0;
}
Clang:
test2(int): # @test2(int)
xor eax, eax
test edi, 2147483647
setne al
ret
GCC:
test3(int):
xor eax, eax
add edi, edi
setne al
ret</pre>
</div>
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