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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] Illegal vector length reductions cause partial scalarization"
href="https://bugs.llvm.org/show_bug.cgi?id=44478">44478</a>
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<tr>
<th>Summary</th>
<td>[X86] Illegal vector length reductions cause partial scalarization
</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>
<tr>
<th>OS</th>
<td>Windows NT
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<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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<div>
<pre><a href="https://gcc.godbolt.org/z/zbkZUp">https://gcc.godbolt.org/z/zbkZUp</a>
clang -g0 -O3 -march=btver2
float sum4(int *x) {
return x[0] + x[1] + x[2] + x[3];
}
float sum5(int *x) {
return x[0] + x[1] + x[2] + x[3] + x[4];
}
_Z4sum4Pi: # @_Z4sum4Pi
vmovdqu (%rdi), %xmm0
vphaddd %xmm0, %xmm0, %xmm0
vphaddd %xmm0, %xmm0, %xmm0
vcvtdq2ps %xmm0, %xmm0
retq
_Z4sum5Pi: # @_Z4sum5Pi
vmovdqu (%rdi), %xmm0
vphaddd %xmm0, %xmm0, %xmm0
vphaddd %xmm0, %xmm0, %xmm0
vmovd %xmm0, %eax
addl 16(%rdi), %eax
vcvtsi2ss %eax, %xmm1, %xmm0
retq
It'd probably be better to keep sum5 etc. entirely on the vector unit and avoid
fpu<->gpu traffic
_Z4sum5Pi: # @_Z4sum5Pi
vmovdqu (%rdi), %xmm0
vphaddd %xmm0, %xmm0, %xmm0
vphaddd %xmm0, %xmm0, %xmm0
vmovd 16(%rdi), %xmm1
vpaddd %xmm1, %xmm0, %xmm0
vcvtdq2ps %xmm0, %xmm0
retq</pre>
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