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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - clang emits unnecessary vinsertf128 and vextractf128"
href="http://llvm.org/bugs/show_bug.cgi?id=15487">15487</a>
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</tr>
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<th>Summary</th>
<td>clang emits unnecessary vinsertf128 and vextractf128
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>quality-of-implementation
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kretz@kde.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Compile the following testcase with "clang++ -O3 -mavx -c":
#include <immintrin.h>
static inline __m256i __attribute__((always_inline)) my_add(__m256i a0, __m256i
b0)
{
__m128i a1 = _mm256_extractf128_si256(a0, 1);
__m128i b1 = _mm256_extractf128_si256(b0, 1);
__m256i r =
_mm256_castsi128_si256(_mm_add_epi32(_mm256_castsi256_si128(a0),
_mm256_castsi256_si128(b0)));
r = _mm256_insertf128_si256(r, _mm_add_epi32(a1, b1), 1);
return r;
}
extern int DATA[];
void use_insert_extract()
{
__m256i x = _mm256_loadu_si256((__m256i*)&DATA[0]);
__m256i y = _mm256_loadu_si256((__m256i*)&DATA[1]);
x = my_add(x, y);
x = my_add(x, y);
_mm256_storeu_si256((__m256i*)&DATA[0], x);
}
Notice the following missing optimizations:
1. clang issues a vmovups + vinsertf128 for the two unaligned-load intrinsics.
Seeing the following vextractf128 it could optimze it though. I.e. this is what
I currently get:
vmovups 0x0(%rip),%xmm0 # DATA
vinsertf128 $0x1,0x0(%rip),%ymm0,%ymm1 # DATA+0xf
vextractf128 $0x1,%ymm1,%xmm1
instead clang could optimize to:
vmovups 0x0(%rip),%xmm0 # DATA
vmovups 0x0(%rip),%ymm1 # DATA+0xf
2. clang issues two unnecessary vinsertf128 which result from the construction
of the r variable in my_add. The compiler already recognizes that the
vextractf128 in this case can be skipped, but the vinsertf128 is still kept.
I.e. this is what I currently get:
vpaddd %xmm1,%xmm3,%xmm3
vpaddd %xmm1,%xmm3,%xmm1
vpaddd %xmm0,%xmm2,%xmm4
vxorps %xmm2,%xmm2,%xmm2
vinsertf128 $0x0,%xmm4,%ymm2,%ymm4
vinsertf128 $0x1,%xmm3,%ymm4,%ymm3
vpaddd %xmm0,%xmm3,%xmm0
vinsertf128 $0x0,%xmm0,%ymm2,%ymm0
vinsertf128 $0x1,%xmm1,%ymm0,%ymm0
instead clang could optimize this to:
vpaddd %xmm1,%xmm3,%xmm3
vpaddd %xmm1,%xmm3,%xmm1
vpaddd %xmm0,%xmm2,%xmm2
vpaddd %xmm0,%xmm2,%xmm0
vinsertf128 $0x1,%xmm1,%ymm0,%ymm0</pre>
</div>
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