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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 - Unnecessary use of SIMD"
href="https://bugs.llvm.org/show_bug.cgi?id=44781">44781</a>
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<th>Summary</th>
<td>Unnecessary use of SIMD
</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>All
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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>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>kobalicek.petr@gmail.com
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</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
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<pre>Clang tries sometimes so hard to use SIMD that it generates worse code than
actually not using it. A very simple example below:
struct Box {
int x0, y0, x1, y1;
};
bool check(const Box& box) noexcept {
return ((box.x0 | box.y0 | box.x1 | box.y1) & 0xFFu) == 0;
}
Clang 8+ compiles this code to:
check(Box const&):
vmovdqu xmm0, xmmword ptr [rdi]
vpshufd xmm1, xmm0, 78 # xmm1 = xmm0[2,3,0,1]
vpor xmm0, xmm0, xmm1
vpshufd xmm1, xmm0, 229 # xmm1 = xmm0[1,1,2,3]
vpor xmm0, xmm0, xmm1
vpextrb eax, xmm0, 0
test al, al
sete al
ret
Whereas GCC (and Clang 7 and lesser) is just really fine with a scalar code:
check(Box const&):
mov eax, DWORD PTR [rdi]
or eax, DWORD PTR [rdi+4]
or eax, DWORD PTR [rdi+8]
or eax, DWORD PTR [rdi+12]
test al, al
sete al
ret
The resulting SIMD code Clang produces is just worse (and bigger) than the
scalar one, and I think it would also be slower as it contains shuffles and
extract.</pre>
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