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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86][SSE] extra pshufhw generated in simple shuffle code"
href="https://bugs.llvm.org/show_bug.cgi?id=34686">34686</a>
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<th>Summary</th>
<td>[X86][SSE] extra pshufhw generated in simple shuffle code
</td>
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<th>Product</th>
<td>new-bugs
</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>All
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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>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>cox2@cooper.edu
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<pre>Trunk turns these two shuffles into three (The commented out line is equivalent
and generates the same 3 shuffles):
#include <emmintrin.h>
__m128i m(__m128i i) {
const __m128i a = _mm_shufflelo_epi16(i, 0x14);
return _mm_shuffle_epi32(a, 0x44);
// return _mm_unpacklo_epi64(a, a);
}
clang -S -O3 test.c -o -
m:
pshufd $196, %xmm0, %xmm0 # xmm0 = xmm0[0,1,0,3]
pshuflw $20, %xmm0, %xmm0 # xmm0 = xmm0[0,1,1,0,4,5,6,7]
pshufhw $20, %xmm0, %xmm0 # xmm0 = xmm0[0,1,2,3,4,5,5,4]
retq
This can be observed at all optimization levels above 0.
I'd expect the same output that gcc generates:
gcc -S -O3 test.c -o -
m:
pshuflw $20, %xmm0, %xmm0
pshufd $68, %xmm0, %xmm0
ret</pre>
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