[InstCombine] fold shuffle-with-binop and common value
This is the last significant change suggested in PR37806:
https://bugs.llvm.org/show_bug.cgi?id=37806#c5
...though there are several follow-ups noted in the code comments
in this patch to complete this transform.
It's possible that a binop feeding a select-shuffle has been eliminated
by earlier transforms (or the code was just written like this in the 1st
place), so we'll fail to match the patterns that have 2 binops from:
D48401,
D48678,
D48662,
D48485.
In that case, we can try to materialize identity constants for the remaining
binop to fill in the "ghost" lanes of the vector (where we just want to pass
through the original values of the source operand).
I added comments to ConstantExpr::getBinOpIdentity() to show planned follow-ups.
For now, we only handle the 5 commutative integer binops (add/mul/and/or/xor).
Differential Revision: https://reviews.llvm.org/D48830
llvm-svn: 336196
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index b06889f..e3b935c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1182,6 +1182,50 @@
return {};
}
+static Instruction *foldSelectShuffleWith1Binop(ShuffleVectorInst &Shuf) {
+ assert(Shuf.isSelect() && "Must have select-equivalent shuffle");
+
+ // Are we shuffling together some value and that same value after it has been
+ // modified by a binop with a constant?
+ Value *Op0 = Shuf.getOperand(0), *Op1 = Shuf.getOperand(1);
+ Constant *C;
+ bool Op0IsBinop;
+ if (match(Op0, m_BinOp(m_Specific(Op1), m_Constant(C))))
+ Op0IsBinop = true;
+ else if (match(Op1, m_BinOp(m_Specific(Op0), m_Constant(C))))
+ Op0IsBinop = false;
+ else
+ return nullptr;
+
+ auto *BO = cast<BinaryOperator>(Op0IsBinop ? Op0 : Op1);
+ Value *X = Op0IsBinop ? Op1 : Op0;
+ // TODO: Allow div/rem by accounting for potential UB due to undef elements.
+ if (BO->isIntDivRem())
+ return nullptr;
+
+ // The identity constant for a binop leaves a variable operand unchanged. For
+ // a vector, this is a splat of something like 0, -1, or 1.
+ // If there's no identity constant for this binop, we're done.
+ BinaryOperator::BinaryOps BOpcode = BO->getOpcode();
+ Constant *IdC = ConstantExpr::getBinOpIdentity(BOpcode, Shuf.getType());
+ if (!IdC)
+ return nullptr;
+
+ // Shuffle identity constants into the lanes that return the original value.
+ // Example: shuf (mul X, {-1,-2,-3,-4}), X, {0,5,6,3} --> mul X, {-1,1,1,-4}
+ // Example: shuf X, (add X, {-1,-2,-3,-4}), {0,1,6,7} --> add X, {0,0,-3,-4}
+ // The existing binop constant vector remains in the same operand position.
+ Constant *Mask = Shuf.getMask();
+ Constant *NewC = Op0IsBinop ? ConstantExpr::getShuffleVector(C, IdC, Mask) :
+ ConstantExpr::getShuffleVector(IdC, C, Mask);
+
+ // shuf (bop X, C), X, M --> bop X, C'
+ // shuf X, (bop X, C), M --> bop X, C'
+ Instruction *NewBO = BinaryOperator::Create(BOpcode, X, NewC);
+ NewBO->copyIRFlags(BO);
+ return NewBO;
+}
+
/// Try to fold shuffles that are the equivalent of a vector select.
static Instruction *foldSelectShuffle(ShuffleVectorInst &Shuf,
InstCombiner::BuilderTy &Builder,
@@ -1189,6 +1233,9 @@
if (!Shuf.isSelect())
return nullptr;
+ if (Instruction *I = foldSelectShuffleWith1Binop(Shuf))
+ return I;
+
BinaryOperator *B0, *B1;
if (!match(Shuf.getOperand(0), m_BinOp(B0)) ||
!match(Shuf.getOperand(1), m_BinOp(B1)))