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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [X86][SSE] Failure to split vector loads for scalarized operations"
href="https://llvm.org/bugs/show_bug.cgi?id=30986">30986</a>
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<th>Summary</th>
<td>[X86][SSE] Failure to split vector loads for scalarized operations
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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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>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mkuper@google.com, spatel+llvm@rotateright.com, zvi.rackover@intel.com
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>As discussed on <a href="https://reviews.llvm.org/D26521">https://reviews.llvm.org/D26521</a>.
Scalarized cases fail to split any vector loads, resulting in a lot of extra
(potentially very slow) vector -> gpr traffic:
clang -S -O3 -march=btver2
#include <x86intrin.h>
__m128i popcnt1(__m128i *in) {
return (__m128i) {
__builtin_popcountll(in[0][0]),
__builtin_popcountll(in[0][1]) };
}
popcnt1(long long __vector(2)*):
vmovdqu (%rdi), %xmm0
vmovq %xmm0, %rax
vpextrq $1, %xmm0, %rcx
popcntq %rax, %rax
popcntq %rcx, %rcx
vmovq %rcx, %xmm0
vmovq %rax, %xmm1
vpunpcklqdq %xmm0, %xmm1, %xmm0 # xmm0 = xmm1[0],xmm0[0]
retq
It would be better as:
popcnt1(long long __vector(2)*):
popcntq (%rdi), %rax
popcntq 8(%rdi), %rcx
vmovq %rcx, %xmm0
vmovq %rax, %xmm1
vpunpcklqdq %xmm0, %xmm1, %xmm0 # xmm0 = xmm1[0],xmm0[0]
retq
Something similar happens when the source vector is spilled - it is first
restored to a vector register and then transferred to gprs. In some cases after
the results are transferred back to the vector register and then spilled again
- we could have spilled the scalar values in vector order directly...</pre>
</div>
</p>
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