[InstCombine] recognize bswap disguised as shufflevector
bitcast <N x i8> (shuf X, undef, <N, N-1,...0>) to i{N*8} --> bswap (bitcast X to i{N*8})
In PR43146:
https://bugs.llvm.org/show_bug.cgi?id=43146
...we have a more complicated case where SLP is making a mess of bswap. This patch won't
do anything for that currently, but we need to improve bswap recognition in instcombine,
SLP, and/or a standalone pass to avoid that problem.
This is limited using the data-layout so we don't try to do this transform with actual
vector types. The backend does not appear to have folds to convert in either direction,
so we don't want to mess up something that is actually better lowered as a shuffle.
On x86, we're trading something like this:
vmovd %edi, %xmm0
vpshufb LCPI0_0(%rip), %xmm0, %xmm0 ## xmm0 = xmm0[3,2,1,0,u,u,u,u,u,u,u,u,u,u,u,u]
vmovd %xmm0, %eax
For:
movl %edi, %eax
bswapl %eax
Differential Revision: https://reviews.llvm.org/D66965
llvm-svn: 370659
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 4be44da..1a569d2 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -2416,6 +2416,22 @@
return new ShuffleVectorInst(LHS, RHS, Shuf->getOperand(2));
}
}
+
+ // A bitcasted-to-scalar and byte-reversing shuffle is better recognized as
+ // a byte-swap:
+ // bitcast <N x i8> (shuf X, undef, <N, N-1,...0>) --> bswap (bitcast X)
+ // TODO: We should match the related pattern for bitreverse.
+ if (DestTy->isIntegerTy() &&
+ DL.isLegalInteger(DestTy->getScalarSizeInBits()) &&
+ SrcTy->getScalarSizeInBits() == 8 && NumShufElts % 2 == 0 &&
+ Shuf->hasOneUse() && Shuf->isReverse()) {
+ assert(ShufOp0->getType() == SrcTy && "Unexpected shuffle mask");
+ assert(isa<UndefValue>(ShufOp1) && "Unexpected shuffle op");
+ Function *Bswap =
+ Intrinsic::getDeclaration(CI.getModule(), Intrinsic::bswap, DestTy);
+ Value *ScalarX = Builder.CreateBitCast(ShufOp0, DestTy);
+ return IntrinsicInst::Create(Bswap, { ScalarX });
+ }
}
// Handle the A->B->A cast, and there is an intervening PHI node.