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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 opportunity to use cmp with exx instead od rxx (code size optimization)"
href="https://bugs.llvm.org/show_bug.cgi?id=43823">43823</a>
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<th>Summary</th>
<td>Missed opportunity to use cmp with exx instead od rxx (code size optimization)
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<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>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
</td>
</tr></table>
<p>
<div>
<pre>bool test1(U64 val)
{
return ((U32)(val & 0x120)) == 0x20;
}
define dso_local zeroext i1 @test1(i64 %0) local_unnamed_addr #0 {
%2 = and i64 %0, 288
%3 = icmp eq i64 %2, 32
ret i1 %3
}
test1:
and edi, 288
cmp rdi, 32
sete al
ret
(-O0's trunc LLVM IR instruction is eliminated)
but codegen could use "cmp edi" instead of "cmp rdi":
test1:
and edi, 288
cmp edi, 32
sete al
ret
GCC does this code size optimization (eliminate REX prefix), Clang does not.
Another opportunity for "cmp edi":
bool test2(U64 val)
{
return (val & 0x120) == 0x20;
}
test2:
and edi, 288
cmp rdi, 32
sete al
ret
Both GCC and Clang fails to use rdi for "test2" case.
<a href="https://godbolt.org/z/A0hjTg">https://godbolt.org/z/A0hjTg</a>
Another case:
bool test3(U64 val)
{
return ((short)(val & 0x120)) == 0x20;
}
Clang
test3: # @test3
and edi, 288
cmp rdi, 32
sete al
ret
GCC
test3:
and di, 288
cmp di, 32
sete al
ret</pre>
</div>
</p>
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