[InstCombine] fold insertelement into splat of same scalar
Forming the canonical splat shuffle improves analysis and
may allow follow-on transforms (although some possibilities
are missing as shown in the test diffs).
The backend generically turns these patterns into build_vector,
so there should be no codegen regressions. All targets are
expected to be able to lower splats efficiently.
llvm-svn: 365379
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index b928ae5..dc9abdd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -732,6 +732,40 @@
return new ShuffleVectorInst(FirstIE, UndefVec, ConstantVector::get(Mask));
}
+/// Try to fold an insert element into an existing splat shuffle by changing
+/// the shuffle's mask to include the index of this insert element.
+static Instruction *foldInsEltIntoSplat(InsertElementInst &InsElt) {
+ // Check if the vector operand of this insert is a canonical splat shuffle.
+ auto *Shuf = dyn_cast<ShuffleVectorInst>(InsElt.getOperand(0));
+ if (!Shuf || !Shuf->isZeroEltSplat())
+ return nullptr;
+
+ // Check for a constant insertion index.
+ uint64_t IdxC;
+ if (!match(InsElt.getOperand(2), m_ConstantInt(IdxC)))
+ return nullptr;
+
+ // Check if the splat shuffle's input is the same as this insert's scalar op.
+ Value *X = InsElt.getOperand(1);
+ Value *Op0 = Shuf->getOperand(0);
+ if (!match(Op0, m_InsertElement(m_Undef(), m_Specific(X), m_ZeroInt())))
+ return nullptr;
+
+ // Replace the shuffle mask element at the index of this insert with a zero.
+ // For example:
+ // inselt (shuf (inselt undef, X, 0), undef, <0,undef,0,undef>), X, 1
+ // --> shuf (inselt undef, X, 0), undef, <0,0,0,undef>
+ unsigned NumMaskElts = Shuf->getType()->getVectorNumElements();
+ SmallVector<Constant *, 16> NewMaskVec(NumMaskElts);
+ Type *I32Ty = IntegerType::getInt32Ty(Shuf->getContext());
+ Constant *Zero = ConstantInt::getNullValue(I32Ty);
+ for (unsigned i = 0; i != NumMaskElts; ++i)
+ NewMaskVec[i] = i == IdxC ? Zero : Shuf->getMask()->getAggregateElement(i);
+
+ Constant *NewMask = ConstantVector::get(NewMaskVec);
+ return new ShuffleVectorInst(Op0, UndefValue::get(Op0->getType()), NewMask);
+}
+
/// If we have an insertelement instruction feeding into another insertelement
/// and the 2nd is inserting a constant into the vector, canonicalize that
/// constant insertion before the insertion of a variable:
@@ -950,6 +984,9 @@
if (Instruction *Broadcast = foldInsSequenceIntoSplat(IE))
return Broadcast;
+ if (Instruction *Splat = foldInsEltIntoSplat(IE))
+ return Splat;
+
return nullptr;
}