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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 - [X86][AVX] Regression in subvector scatter due to splitting ymm loads"
href="https://bugs.llvm.org/show_bug.cgi?id=42305">42305</a>
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<th>Summary</th>
<td>[X86][AVX] Regression in subvector scatter due to splitting ymm loads
</td>
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<th>Product</th>
<td>libraries
</td>
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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>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
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
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<th>CC</th>
<td>andrea.dibiagio@gmail.com, craig.topper@gmail.com, greg.bedwell@sony.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre><a href="https://godbolt.org/z/8Cjr7X">https://godbolt.org/z/8Cjr7X</a>
#include <x86intrin.h>
void scatter_subvectors( int count, const float* src, const int* idx, float*
dst )
{
for( int i = 0; i != count; ++i )
{
__m256 j01 = _mm256_loadu_ps((const float*)(src + 0));
__m256 j23 = _mm256_loadu_ps((const float*)(src + 8));
src += 16;
_mm_storeu_ps(dst + idx[0], _mm256_extractf128_ps(j01, 0));
_mm_storeu_ps(dst + idx[1], _mm256_extractf128_ps(j01, 1));
_mm_storeu_ps(dst + idx[2], _mm256_extractf128_ps(j23, 0));
_mm_storeu_ps(dst + idx[3], _mm256_extractf128_ps(j23, 1));
idx += 4;
}
}
We're seeing a regression as the ymm loads are each being split into 2 xmm
loads because we extract+store the subvectors independently. But because
vextractf128 can store directly we're increasing instruction count completely
unnecessarily.</pre>
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