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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - bad code for __builtin_parity on x86 and ARM"
href="https://bugs.llvm.org/show_bug.cgi?id=47433">47433</a>
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<th>Summary</th>
<td>bad code for __builtin_parity on x86 and ARM
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>10.0
</td>
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<th>Hardware</th>
<td>Other
</td>
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<th>OS</th>
<td>FreeBSD
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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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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</tr>
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<th>Reporter</th>
<td>fuzxxl@gmail.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>I have found that __builtin_parity generates poor code on ARM (soft float).
extern int foo(int x) {
return __builtin_parity(x);
}
compiles to
mov r1, #85
and r1, r1, r0, lsr #1
sub r0, r0, r1
movw r1, #13107
movt r1, #13107
and r2, r1, r0, lsr #2
and r0, r0, r1
add r0, r0, r2
movw r1, #3855
movt r1, #271
add r0, r0, r0, lsr #4
and r0, r0, r1
movw r1, #257
movt r1, #257
mul r0, r0, r1
ubfx r0, r0, #24, #1
bx lr
which essentially performs a population count and then truncates to one bit.
This seems awfully suboptimal. Why not a series of shifts and xors?
eor r0, r0, r0, lsr #16
eor r0, r0, r0, lsr #8
eor r0, r0, r0, lsr #4
eor r0, r0, r0, lsr #2
eor r0, r0, r0, lsr #1
and r0, r0, #1
This seems a lot better. The builtin could also recognise if the input has a
width of less than 32 bit and perform less reductions if possible.</pre>
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