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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 --- - _mm_movehl_ps pesimised to movaps/shufpd instead of movhlps"
href="https://llvm.org/bugs/show_bug.cgi?id=26491">26491</a>
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<th>Summary</th>
<td>_mm_movehl_ps pesimised to movaps/shufpd instead of movhlps
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.7
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>performance
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>peter@cordes.ca
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>clang sometimes uses a different instruction that the "normal" one for the
intrinsic. Usually it gets an improvement, but not in this case where it ends
up wasting a movaps and using a bigger instruction.
float hsum_ps(__m128 v) {
__m128 tmp = _mm_movehdup_ps(v); // dest of MOVSHDUP is write-only
__m128 sums = _mm_add_ps(v, tmp);
tmp = _mm_movehl_ps(tmp, sums);
sums = _mm_add_ss(sums, tmp);
return _mm_cvtss_f32(sums);
}
compiles to: clang 3.7.1 -O3 -march=core2
movshdup xmm1, xmm0 # xmm1 = xmm0[1,1,3,3]
addps xmm1, xmm0
movaps xmm0, xmm1
shufpd xmm0, xmm0, 1 # xmm0 = xmm0[1,0]
addss xmm0, xmm1
ret
movhlps is very compact: OF 12 /r, vs. shufpd being a 5 byte insn. At least
it doesn't use shufps, which is slow on Core2. A similar sequence after a loop
produced a movaps/unpckhpd instead of the movhlps.
What I was hoping for (and which gcc produces) is the obvious literal
translation from intrinsics to instructions:
movshdup xmm1, xmm0
addps xmm0, xmm1
movhlps xmm1, xmm0
addss xmm0, xmm1
ret
I used movshdup first because its destination is write-only. I can avoid a
movaps without needing the compiler to keep some other variable live so I can
movhlps into it. (And so I could put it in a function that only takes one
parameter.) I can't use _mm_undefined_ps() without risk of denormals or NaN
slowdowns.</pre>
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</p>
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