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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [x86] missed optimizations for shift-counts: (32-n) can be (-n)"
href="https://bugs.llvm.org/show_bug.cgi?id=34642">34642</a>
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<th>Summary</th>
<td>[x86] missed optimizations for shift-counts: (32-n) can be (-n)
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>peter@cordes.ca
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>unsigned shift(unsigned a, unsigned n){
n = (32-n);
// n &= 31;
return a << n;
}
// <a href="https://godbolt.org/g/YFKxQw">https://godbolt.org/g/YFKxQw</a>
// clang version 6.0.0 (trunk 313348) -march=skylake -O3
mov eax, 32 # -mno-bmi2 version is essentially the same
sub eax, esi
shlx eax, edi, eax
ret
But with n &= 31; we get the obviously-better
neg esi
shlx eax, edi, esi
ret
(shlx masks the shift count identically to how shl r,cl does, with &31 for
32-bit operand size. -mno-bmi2 code-gen is equivalent.)
So clang / llvm doesn't take full advantage of the masking behaviour of x86
shift counts for shlx, shl r,cl, or presumably bt / bts (which do also wrap
around at the destination register size with a register destination.)</pre>
</div>
</p>
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