[InstCombine] narrow vector select with padded condition and extracted result (PR38691)
shuf (sel (shuf NarrowCond, undef, WideMask), X, Y), undef, NarrowMask) -->
sel NarrowCond, (shuf X, undef, NarrowMask), (shuf Y, undef, NarrowMask)
The motivating case from:
https://bugs.llvm.org/show_bug.cgi?id=38691
...is the last regression test. In that case, we're just left with the narrow select.
Note that if we do create new shuffles, they use the existing extraction identity mask,
so there's no danger that this transform creates arbitrary shuffles.
Differential Revision: https://reviews.llvm.org/D51496
llvm-svn: 341708
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index b415c83..d40fa9b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1350,6 +1350,41 @@
return NewBO;
}
+/// Match a shuffle-select-shuffle pattern where the shuffles are widening and
+/// narrowing (concatenating with undef and extracting back to the original
+/// length). This allows replacing the wide select with a narrow select.
+Instruction *narrowVectorSelect(ShuffleVectorInst &Shuf,
+ InstCombiner::BuilderTy &Builder) {
+ // This must be a narrowing identity shuffle. It extracts the 1st N elements
+ // of the 1st vector operand of a shuffle.
+ if (!match(Shuf.getOperand(1), m_Undef()) || !Shuf.isIdentityWithExtract())
+ return nullptr;
+
+ // The vector being shuffled must be a vector select that we can eliminate.
+ // TODO: The one-use requirement could be eased if X and/or Y are constants.
+ Value *Cond, *X, *Y;
+ if (!match(Shuf.getOperand(0),
+ m_OneUse(m_Select(m_Value(Cond), m_Value(X), m_Value(Y)))))
+ return nullptr;
+
+ // We need a narrow condition value. It must be extended with undef elements
+ // and have the same number of elements as this shuffle.
+ unsigned NarrowNumElts = Shuf.getType()->getVectorNumElements();
+ Value *NarrowCond;
+ if (!match(Cond, m_OneUse(m_ShuffleVector(m_Value(NarrowCond), m_Undef(),
+ m_Constant()))) ||
+ NarrowCond->getType()->getVectorNumElements() != NarrowNumElts ||
+ !cast<ShuffleVectorInst>(Cond)->isIdentityWithPadding())
+ return nullptr;
+
+ // shuf (sel (shuf NarrowCond, undef, WideMask), X, Y), undef, NarrowMask) -->
+ // sel NarrowCond, (shuf X, undef, NarrowMask), (shuf Y, undef, NarrowMask)
+ Value *Undef = UndefValue::get(X->getType());
+ Value *NarrowX = Builder.CreateShuffleVector(X, Undef, Shuf.getMask());
+ Value *NarrowY = Builder.CreateShuffleVector(Y, Undef, Shuf.getMask());
+ return SelectInst::Create(NarrowCond, NarrowX, NarrowY);
+}
+
Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
Value *LHS = SVI.getOperand(0);
Value *RHS = SVI.getOperand(1);
@@ -1360,6 +1395,9 @@
if (Instruction *I = foldSelectShuffle(SVI, Builder, DL))
return I;
+ if (Instruction *I = narrowVectorSelect(SVI, Builder))
+ return I;
+
unsigned VWidth = SVI.getType()->getVectorNumElements();
APInt UndefElts(VWidth, 0);
APInt AllOnesEltMask(APInt::getAllOnesValue(VWidth));