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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [x86] shuffle lowering creates strange shuffle mask"
href="https://bugs.llvm.org/show_bug.cgi?id=40306">40306</a>
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<th>Summary</th>
<td>[x86] shuffle lowering creates strange shuffle mask
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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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>spatel+llvm@rotateright.com
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</tr>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
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<pre>This manifested in:
<a href="https://reviews.llvm.org/D56281">https://reviews.llvm.org/D56281</a>
...but it's independent of that patch as shown with this example:
define <8 x i16> @shuf_zeros_undefs(<8 x i16> %x) {
%r = shufflevector <8 x i16> zeroinitializer, <8 x i16> %x, <8 x i32> <i32 9,
i32 1, i32 2, i32 3, i32 undef, i32 undef, i32 undef, i32 undef>
ret <8 x i16> %r
}
Clearly, we have undefs in the high elements of the result, but we get this
unexpected pshufb lowering:
$ llc -o - weird_shufb.ll -mattr=avx
vpshufb LCPI0_0(%rip), %xmm0, %xmm0 ## xmm0 =
xmm0[2,3],zero,zero,zero,zero,zero,zero,zero,zero,zero,zero,zero,zero,xmm0[6,7]
-------------------------------------------------------------------------------
That's not a miscompile, but why are the high elements choosing from the input?
Debug output shows that we created this intermediate state:
t12: v8i16 = X86ISD::UNPCKL t17, t2
t19: v8i16 = X86ISD::PSHUFHW t12, Constant:i8<-24>
t20: v4i32 = bitcast t19
t22: v4i32 = X86ISD::PSHUFD t20, Constant:i8<-26>
t23: v8i16 = bitcast t22
t25: v8i16 = X86ISD::PSHUFLW t23, Constant:i8<75>
-------------------------------------------------------------------------------
So this leads to a question that may not be answerable in the DAG or
statically: what is the ideal x86 lowering for that shuffle?
The pshufb is obviously the smallest code, but should we favor a solution that
doesn't need to load anything?
That could be a shift+blend immediate with zero:
vpsrld $16, %xmm0, %xmm0
vpxor %xmm1, %xmm1, %xmm1
vpblendw $1, %xmm0, %xmm1, %xmm0 ## xmm0 =
xmm0[0],xmm1[1,2,3,4,5,6,7]
Or shift+zext:
vpsrld $16, %xmm0, %xmm0
vpmovzxdq %xmm0, %xmm0
Or if we're ok with a load, but just want to avoid pshufb, it could be a
shift+mask:
vpsrld $16, %xmm0, %xmm0
vpand LCPI0_0(%rip), %xmm0, %xmm0</pre>
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