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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] Two simple palignr shuffles become a two pshufbs and an or."
href="https://bugs.llvm.org/show_bug.cgi?id=39387">39387</a>
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<th>Summary</th>
<td>[X86] Two simple palignr shuffles become a two pshufbs and an or.
</td>
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<th>Product</th>
<td>libraries
</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>Windows NT
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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>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>craig.topper@gmail.com
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Given these two shuffles which can be represented by a 2 input palignr followed
by a fake unary palignr.
t19: v16i8 =
vector_shuffle<11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26> t17, t16
t23: v16i8 = vector_shuffle<10,11,12,13,14,15,0,1,2,3,4,5,6,7,8,9> t19,
undef:v16i8
A generic DAG combine turns it into
vector_shuffle<5,6,7,8,9,10,27,28,29,30,31,0,1,2,3,4> t16, t17
which we legalize to 2 pshufbs and an or like this
Creating new node: t136: v16i8 = BUILD_VECTOR Constant:i8<5>, Constant:i8<6>,
Constant:i8<7>, Constant:i8<8>, Constant:i8<9>, Constant:i8<10>,
Constant:i8<-128>, Constant:i8<-128>, Constant:i8<-128>, Constant:i8<-128>,
Constant:i8<-128>, Constant:i8<0>, Constant:i8<1>, Constant:i8<2>, Constant:i8
Creating new node: t137: v16i8 = X86ISD::PSHUFB t16, t136
Creating new node: t138: v16i8 = BUILD_VECTOR Constant:i8<-128>,
Constant:i8<-128>, Constant:i8<-128>, Constant:i8<-128>, Constant:i8<-128>,
Constant:i8<-128>, Constant:i8<11>, Constant:i8<12>, Constant:i8<13>,
Constant:i8<14>, Constant:i8<15>, Constant:i8<-128>, Constant:i8<-128>,
Constant:i8<-1
Creating new node: t139: v16i8 = X86ISD::PSHUFB t17, t138
Creating new node: t140: v16i8 = or t137, t139
and no target specific DAG combine will recover it.
Perhaps we should be less aggressive in isShuffleMaskLegal for byte sizes?</pre>
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