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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Combining MMX with AVX suboptimal"
href="https://llvm.org/bugs/show_bug.cgi?id=29222">29222</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Combining MMX with AVX suboptimal
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.8
</td>
</tr>
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<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kobalicek.petr@gmail.com
</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>The following code:
#include <mmintrin.h>
#include <immintrin.h>
int fn(int x) {
__m64 mm = _mm_set1_pi32(x);
mm = _mm_packs_pi16(mm, mm);
__m128i xmm = _mm_movpi64_epi64(mm);
xmm = _mm_packs_epi16(xmm, xmm);
return _mm_cvtsi128_si32(xmm);
}
Compiled with '-O2 -Wall -mavx2 -m32 -fomit-frame-pointer' produces:
fn(int):
sub esp, 20
vbroadcastss xmm0, dword ptr [esp + 24] # Cool idea, but not
vmovlps qword ptr [esp + 8], xmm0 # in our context.
movq mm0, qword ptr [esp + 8] # !!!
packsswb mm0, mm0
movq qword ptr [esp], mm0 # These moves are
vmovq xmm0, qword ptr [esp] # correct.
vpacksswb xmm0, xmm0, xmm0
vmovd eax, xmm0
add esp, 20
ret
I know that MMX is not used anymore, but I wonder why clang prefers a code-path
that is one instruction longer and contains 2 memory accesses more than a more
straightforward 'punpckldq'.</pre>
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