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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 - SLP vectorizer produces lower performance code"
href="https://bugs.llvm.org/show_bug.cgi?id=48662">48662</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>SLP vectorizer produces lower performance code
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>chfast@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The code just checks if array uint64_t[4] has all zeros.
using u64 = unsigned long;
struct u256
{
u64 w[4];
};
bool iszero(const u256& u)
{
return (u.w[0] == 0) & (u.w[1] == 0) & (u.w[2] == 0) & (u.w[3] == 0);
}
Without SLP vectorization (-fno-slp-vectorize) the compiler produces nice OR
folding:
iszero(u256 const&): # @iszero(u256 const&)
mov rax, qword ptr [rdi + 8]
or rax, qword ptr [rdi]
or rax, qword ptr [rdi + 16]
or rax, qword ptr [rdi + 24]
sete al
ret
With SLP vectorization (default):
iszero(u256 const&): # @iszero(u256 const&)
movdqu xmm0, xmmword ptr [rdi]
movdqu xmm1, xmmword ptr [rdi + 16]
por xmm1, xmm0
pxor xmm0, xmm0
pcmpeqb xmm0, xmm1
pmovmskb eax, xmm0
cmp eax, 65535
sete al
ret
>From llvm-mca the vectorized code may have higher throughput but higher
latency. In my real life usage of similar code I can definitely notice the
performance degradation.
<a href="https://godbolt.org/z/z19EPc">https://godbolt.org/z/z19EPc</a></pre>
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</p>
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