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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86][SSE] Memory fold scalar unary ops with zero register passthrough"
href="https://bugs.llvm.org/show_bug.cgi?id=40359">40359</a>
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<th>Summary</th>
<td>[X86][SSE] Memory fold scalar unary ops with zero register passthrough
</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
</td>
</tr>
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<th>CC</th>
<td>andrea.dibiagio@gmail.com, craig.topper@gmail.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/uPs8BV">https://godbolt.org/z/uPs8BV</a>
We currently do this:
vmovss (%rdi), %xmm0 # xmm0 = mem[0],zero,zero,zero
vmovss (%rsi), %xmm1 # xmm1 = mem[0],zero,zero,zero
vsqrtss %xmm0, %xmm0, %xmm0
vsqrtss %xmm1, %xmm1, %xmm1
vaddss %xmm1, %xmm0, %xmm0
retq
but we can reduce register pressure when we have multiple uses of the zero by
doing this instead, and even when we don't reuse the register there's no
regression.
vxorps %xmm1, %xmm1, %xmm1
vsqrtss (%rdi), %xmm1, %xmm0
vsqrtss (%rsi), %xmm1, %xmm1
vaddss %xmm1, %xmm0, %xmm0
retq
This is really about AVX encoded instructions but I can't see any reason not to
do this on older SSE instructions as well.</pre>
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