1 files changed, 46 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index eea890c815c..f35a6ce2d93 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -296,6 +296,49 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
   return BinaryOperator::Create(Instruction::And, NewShift, NewMask);
 }
 
+/// If we have a shift-by-constant of a bitwise logic op that itself has a
+/// shift-by-constant operand with identical opcode, we may be able to convert
+/// that into 2 independent shifts followed by the logic op. This eliminates a
+/// a use of an intermediate value (reduces dependency chain).
+static Instruction *foldShiftOfShiftedLogic(BinaryOperator &I,
+                                            InstCombiner::BuilderTy &Builder) {
+  assert(I.isShift() && "Expected a shift as input");
+  auto *LogicInst = dyn_cast<BinaryOperator>(I.getOperand(0));
+  if (!LogicInst || !LogicInst->isBitwiseLogicOp() || !LogicInst->hasOneUse())
+    return nullptr;
+
+  const APInt *C0, *C1;
+  if (!match(I.getOperand(1), m_APInt(C1)))
+    return nullptr;
+
+  Instruction::BinaryOps ShiftOpcode = I.getOpcode();
+  Type *Ty = I.getType();
+
+  // Find a matching one-use shift by constant. The fold is not valid if the sum
+  // of the shift values equals or exceeds bitwidth.
+  // TODO: Remove the one-use check if the other logic operand (Y) is constant.
+  Value *X, *Y;
+  auto matchFirstShift = [&](Value *V) {
+    return match(V, m_OneUse(m_Shift(m_Value(X), m_APInt(C0)))) &&
+           cast<BinaryOperator>(V)->getOpcode() == ShiftOpcode &&
+           (*C0 + *C1).ult(Ty->getScalarSizeInBits());
+  };
+
+  // Logic ops are commutative, so check each operand for a match.
+  if (matchFirstShift(LogicInst->getOperand(0)))
+    Y = LogicInst->getOperand(1);
+  else if (matchFirstShift(LogicInst->getOperand(1)))
+    Y = LogicInst->getOperand(0);
+  else
+    return nullptr;
+
+  // shift (logic (shift X, C0), Y), C1 -> logic (shift X, C0+C1), (shift Y, C1)
+  Constant *ShiftSumC = ConstantInt::get(Ty, *C0 + *C1);
+  Value *NewShift1 = Builder.CreateBinOp(ShiftOpcode, X, ShiftSumC);
+  Value *NewShift2 = Builder.CreateBinOp(ShiftOpcode, Y, I.getOperand(1));
+  return BinaryOperator::Create(LogicInst->getOpcode(), NewShift1, NewShift2);
+}
+
 Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   assert(Op0->getType() == Op1->getType());
@@ -348,6 +391,9 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
     return &I;
   }
 
+  if (Instruction *Logic = foldShiftOfShiftedLogic(I, Builder))
+    return Logic;
+
   return nullptr;
 }