<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __m256i (a+b*2)+b (vpmullw;vpaddw) is slower than (b+a*2)+a (3 vpaddw)"
href="https://bugs.llvm.org/show_bug.cgi?id=48078">48078</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>__m256i (a+b*2)+b (vpmullw;vpaddw) is slower than (b+a*2)+a (3 vpaddw)
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>i@maskray.me
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, pengfei.wang@intel.com, spatel+llvm@rotateright.com
</td>
</tr></table>
<p>
<div>
<pre>Discovered by JP Maaninen
#include <immintrin.h>
__m256i Slow(__m256i a, __m256i b) {
__m256i c = _mm256_add_epi16(a, _mm256_slli_epi16(a, 1));
return _mm256_add_epi16(c, b);
}
vpmullw .LCPI0_0(%rip), %ymm0, %ymm0
vpaddw %ymm1, %ymm0, %ymm0
retq
__m256i Fast(__m256i a, __m256i b) {
__m256i c = _mm256_add_epi16(b, _mm256_slli_epi16(a, 1));
return _mm256_add_epi16(c, a);
}
vpaddw %ymm0, %ymm0, %ymm2
vpaddw %ymm0, %ymm1, %ymm0
vpaddw %ymm2, %ymm0, %ymm0
retq
----
This is either instcombine's problem or the backend's lack of optimization.
define dso_local <4 x i64> @_Z4SlowDv4_xS_(<4 x i64> %a, <4 x i64> %b)
local_unnamed_addr #0 {
entry:
%0 = bitcast <4 x i64> %a to <16 x i16>
%1 = shl <16 x i16> %0, <i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16
1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1>
%add.i5 = add <16 x i16> %1, %0
%2 = bitcast <16 x i16> %add.i5 to <4 x i64>
%3 = bitcast <4 x i64> %b to <16 x i16>
%add.i = add <16 x i16> %3, %add.i5
%4 = bitcast <16 x i16> %add.i to <4 x i64>
ret <4 x i64> %4
}
attributes #0 = { norecurse nounwind readnone uwtable
"disable-tail-calls"="false" "frame-pointer"="none"
"less-precise-fpmad"="false" "min-legal-vector-width"="256"
"no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false"
"no-signed-zeros-fp-math"="false" "no-trapping-math"="true"
"stack-protector-buffer-size"="8" "target-cpu"="haswell"
"target-features"="+avx,+avx2,+bmi,+bmi2,+cx16,+cx8,+f16c,+fma,+fsgsbase,+fxsr,+invpcid,+lzcnt,+mmx,+movbe,+pclmul,+popcnt,+rdrnd,+sahf,+sse,+sse2,+sse3,+sse4.1,+sse4.2,+ssse3,+x87,+xsave,+xsaveopt"
"unsafe-fp-math"="false" "use-soft-float"="false" }
opt -passes=instcombine -S generates
%add.i5 = mul <16 x i16> %0, <i16 3, i16 3, i16 3, i16 3, i16 3, i16 3, i16
3, i16 3, i16 3, i16 3, i16 3, i16 3, i16 3, i16 3, i16 3, i16 3>
which will lower to VPMULLWYrm in X86ISelDAGToDAG and sticks after every
codegen pass.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>