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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization to auto-vectorize __builtin_bitreverse"
href="https://bugs.llvm.org/show_bug.cgi?id=50189">50189</a>
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<th>Summary</th>
<td>Missed optimization to auto-vectorize __builtin_bitreverse
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</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>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>scovich@gmail.com
</td>
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>This code:
#include <cstdint>
#define DEFINE_BITREVERSE_LOOP(N) \
void bitreverse##N##_loop(int##N##_t* values, int n) { \
for (int i = 0; i < n; i++) { \
values[i] = __builtin_bitreverse##N(values[i]); \
} \
} \
DEFINE_BITREVERSE_LOOP(8);
DEFINE_BITREVERSE_LOOP(16);
DEFINE_BITREVERSE_LOOP(32);
DEFINE_BITREVERSE_LOOP(64);
When compiled with -O3 -march=armv8.2-a+fp16+rcpc+dotprod+crypto+simd+crc
Both clang-9 and clang-11 produce all kinds of code (sometimes even
auto-vectorized, for bytes), instead of just passing vector registers to the
rbit instruction.
This seems a bit ironic, given that __builtin_bitreverse was originally
invented to support arm rbit.</pre>
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