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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - llvm.bitreverse.i8 on x86 lowers to questionable assembly"
href="https://llvm.org/bugs/show_bug.cgi?id=31810">31810</a>
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<th>Summary</th>
<td>llvm.bitreverse.i8 on x86 lowers to questionable assembly
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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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>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
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<th>Reporter</th>
<td>gonzalobg88@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>On x86 the 32 and 64 bit versions of bitreverse look very good, but the
generated code for the 8-bit version looks worse than the naive implementation:
<a href="https://godbolt.org/g/kAlOC7">https://godbolt.org/g/kAlOC7</a>
@bitrev_8_naive(unsigned char)
mov eax, 2149582850
imul rax, rdi
movabs rcx, 36578664720
and rcx, rax
movabs rax, 4311810305
imul rax, rcx
shr rax, 32
ret
vs llvm.bitreverse.i8:
@bitrev_8(unsigned char)
rol dil, 4
mov eax, edi
and al, 51
shl al, 2
and dil, -52
shr dil, 2
or dil, al
mov eax, edi
and al, 85
add al, al
and dil, -86
shr dil
or dil, al
mov eax, edi
ret</pre>
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