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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 - _mm256_xor_si256 prefers vxorps over vpxor on integer vectors when out-of-context"
href="https://bugs.llvm.org/show_bug.cgi?id=36127">36127</a>
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<th>Summary</th>
<td>_mm256_xor_si256 prefers vxorps over vpxor on integer vectors when out-of-context
</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>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>gonzalobg88@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>This code (see it live: <a href="https://godbolt.org/g/jJHBMi">https://godbolt.org/g/jJHBMi</a>):
#include <immintrin.h>
__attribute__((__always_inline__, __nodebug__, __target__("avx2")))
__m256i foo(__m256i a, __m256i b) {
auto c = _mm256_add_epi64(a, b);
return _mm256_xor_si256(a, c);
}
__attribute__((__always_inline__, __nodebug__, __target__("avx2")))
__m256i bar(__m256i a, __m256i b) {
return _mm256_xor_si256(a, b);
}
generates this assembly:
foo(long long __vector(4), long long __vector(4)): #
@foo(long long __vector(4), long long __vector(4))
push rbp
mov rbp, rsp
and rsp, -32
sub rsp, 32
vmovdqa ymm0, ymmword ptr [rbp + 16]
vpaddq ymm1, ymm0, ymmword ptr [rbp + 48]
vpxor ymm0, ymm1, ymm0
mov rsp, rbp
pop rbp
ret
bar(long long __vector(4), long long __vector(4)): #
@bar(long long __vector(4), long long __vector(4))
push rbp
mov rbp, rsp
and rsp, -32
sub rsp, 32
vmovaps ymm0, ymmword ptr [rbp + 48]
vxorps ymm0, ymm0, ymmword ptr [rbp + 16]
mov rsp, rbp
pop rbp
ret
So it looks to me that LLVM/clang are choosing `vxorps` by default, and only
`vpxor` if the vectors are being already operated on in the integer domain and
switching domains would increase latency (foo).
However, even though `vxorps` has a smaller encoding, we are operating on
integer vectors here in both cases (foo and bar), and given that `vxorps` and
`vpxor` have the same latency, but that `vpxor` has Nx times the throughput of
`vxorps` (where N is in range [3, 5]), LLVM should be emitting `vpxor` here for
bar as well unless we are optimizing for code-size which the example above
doesn't do.
Of course, when already operating in the floating point domain, LLVM should
continue to emit `vxorps` to avoid increasing latency by switching domains.</pre>
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