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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 - [PCG] Poor shuffle lane tracking"
href="https://bugs.llvm.org/show_bug.cgi?id=35454">35454</a>
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<th>Summary</th>
<td>[PCG] Poor shuffle lane tracking
</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>
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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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>konstantin.belochapka@sony.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>the compiler produces suboptimal code in the following cases:
///
__m128i laneTest1_0( __m128i v )
{
v = _mm_shuffle_epi32( v, _MM_SHUFFLE(0,1,2,3) ); // mirror lanes
v = _mm_add_epi8( v, v );
v = _mm_shuffle_epi32( v, _MM_SHUFFLE(0,1,2,3) ); // mirror lanes
return v;
}
vpshufd $27, %xmm0, %xmm0 # xmm0 = xmm0[3,2,1,0]
vpaddb %xmm0, %xmm0, %xmm0
vpshufd $27, %xmm0, %xmm0 # xmm0 = xmm0[3,2,1,0]
retq
__m128i laneTest1_1( __m128i v )
{
v = _mm_shuffle_epi32( v, _MM_SHUFFLE(0,1,2,3) ); // mirror lanes
v = _mm_add_epi16( v, v );
v = _mm_shuffle_epi32( v, _MM_SHUFFLE(0,1,2,3) ); // mirror lanes
return v;
}
vpshufd $27, %xmm0, %xmm0 # xmm0 = xmm0[3,2,1,0]
vpaddw %xmm0, %xmm0, %xmm0
vpshufd $27, %xmm0, %xmm0 # xmm0 = xmm0[3,2,1,0]
retq
In both cases, the shuffles could be optimized out.
Note that this happens only when the shuffle type doesn't match the
arithmetic/binop operation.
For example, the above test cases, shuffles work on packed 32-bit integer
values, while the vector add instructions work on packed 8/16 bits values.</pre>
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</p>
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