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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] Terrible codegen for 'splatting' memory copies"
href="https://bugs.llvm.org/show_bug.cgi?id=42024">42024</a>
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<th>Summary</th>
<td>[X86] Terrible codegen for 'splatting' memory copies
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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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>anton.a.afanasyev@gmail.com, craig.topper@gmail.com, lebedev.ri@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, nikita.ppv@gmail.com, spatel+llvm@rotateright.com
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<pre>I'm reporting this in X86 as we may be able to resolve much of it with better
costs and/or shuffle combining but the issue probably needs some fixes in the
vectorizers - we can reassign as necessary.
By splatting memory copy I mean a 1:N copy of each element, e.g. from a
grayscale to RGB uint8_t image copy:
template <typename T>
void splat3_copy(size_t n, T* __restrict dst, const T* __restrict src) {
for (size_t i = 0; i < n; ++i) {
auto s = src[i];
dst[3*i+0] = s;
dst[3*i+1] = s;
dst[3*i+2] = s;
}
}
void splat3_copy8(size_t h, size_t w, unsigned char* __restrict dst, const
unsigned char* __restrict src) {
splat3_copy(h * w, dst, src);
}
<a href="https://gcc.godbolt.org/z/8X-TrN">https://gcc.godbolt.org/z/8X-TrN</a>
btver2 (AVX1) only uses individual byte loads + stores, while haswell (AVX2)
generates multiple vector loops of different sizes (with some pretty horrible
shuffles for what should be simple broadcasts+pshufb).</pre>
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