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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Redundant zeroing of high vector elements for scalar SSE/AVX instructions"
href="http://llvm.org/bugs/show_bug.cgi?id=22788">22788</a>
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<th>Summary</th>
<td>Redundant zeroing of high vector elements for scalar SSE/AVX instructions
</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>
</tr>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>michael.m.kuperstein@intel.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Using scalar x86 intrinsics results in IR similar to this:
define void @foo(double* %p, double* %q, <2 x double>* %r) {
%v = load double, double* %p, align 16
%w = load double, double* %q, align 16
%mul = fmul double %v, %w
%vec = insertelement <2 x double> <double undef, double 0.000000e+00>, double
%mul, i32 0
store <2 x double> %vec, <2 x double>* %r, align 16
ret void
}
This gets lowered into:
movsd (%rcx), %xmm0 # xmm0 = mem[0],zero
mulsd (%rdx), %xmm0
movq %xmm0, %xmm0 # xmm0 = xmm0[0],zero
movdqa %xmm0, (%r8)
The movq is redundant - movsd from memory zeroes the high element, and mulsd
preserves it.</pre>
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</p>
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