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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 --- - Missed optimisation - horizontal max for vectors is not optimised."
href="https://llvm.org/bugs/show_bug.cgi?id=23116">23116</a>
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<tr>
<th>Summary</th>
<td>Missed optimisation - horizontal max for vectors is not optimised.
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nick@indigorenderer.com
</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>Computing the horizontal max (or min etc..) of a vector is not optimised to
faster code.
For example, the C code:
#include <immintrin.h>
inline float max(float a, float b)
{
return a > b ? a : b;
}
float findMax(__m256 v)
{
return max(max(max(max(max(max(max(v[0], v[1]), v[2]), v[3]), v[4]),
v[5]), v[6]), v[7]);
}
Is compiled to by Clang 3.7/trunk to:
findMax(float __vector(8)): # @findMax(float
__vector(8))
vmovshdup %xmm0, %xmm1 # xmm1 = xmm0[1,1,3,3]
vmaxss %xmm1, %xmm0, %xmm1
vpermilpd $1, %xmm0, %xmm2 # xmm2 = xmm0[1,0]
vmaxss %xmm2, %xmm1, %xmm1
vpermilps $231, %xmm0, %xmm2 # xmm2 = xmm0[3,1,2,3]
vmaxss %xmm2, %xmm1, %xmm1
vextractf128 $1, %ymm0, %xmm0
vmaxss %xmm0, %xmm1, %xmm1
vmovshdup %xmm0, %xmm2 # xmm2 = xmm0[1,1,3,3]
vmaxss %xmm2, %xmm1, %xmm1
vpermilpd $1, %xmm0, %xmm2 # xmm2 = xmm0[1,0]
vmaxss %xmm2, %xmm1, %xmm1
vpermilps $231, %xmm0, %xmm0 # xmm0 = xmm0[3,1,2,3]
vmaxss %xmm0, %xmm1, %xmm0
vzeroupper
retq
Which is basically 7 vmaxss's in serial (slow).
It could be optimised to something like:
float findMax(__m256 v)
{
__m128 a = _mm256_extractf128_ps(v, 0);
__m128 b = _mm256_extractf128_ps(v, 1);
__m128 c = _mm_max_ps(a, b);
return max(max(c[0], c[1]), max(c[2], c[3]));
}
Which compiles to:
findMax(float __vector(8)): # @findMax(float
__vector(8))
vextractf128 $1, %ymm0, %xmm1
vmaxps %xmm1, %xmm0, %xmm0
vmovshdup %xmm0, %xmm1 # xmm1 = xmm0[1,1,3,3]
vmaxss %xmm1, %xmm0, %xmm1
vpermilpd $1, %xmm0, %xmm2 # xmm2 = xmm0[1,0]
vpermilps $231, %xmm0, %xmm0 # xmm0 = xmm0[3,1,2,3]
vmaxss %xmm0, %xmm2, %xmm0
vmaxss %xmm0, %xmm1, %xmm0
vzeroupper
retq
See <a href="http://goo.gl/jM3KNz">http://goo.gl/jM3KNz</a> for the code.</pre>
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