[InstCombine] Support pulling left shifts through a subtract with constant LHS
We already support pulling through an add with constant RHS. We can do the same for subtract.
Differential Revision: https://reviews.llvm.org/D36443
llvm-svn: 310407
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 5f21666..098079a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -510,6 +510,20 @@
NewRHS);
}
}
+
+ // If the operand is a subtract with a constant LHS, and the shift
+ // is the only use, we can pull it out of the shift.
+ // This folds (shl (sub C1, X), C2) -> (sub (C1 << C2), (shl X, C2))
+ if (isLeftShift && Op0BO->getOpcode() == Instruction::Sub &&
+ match(Op0BO->getOperand(0), m_APInt(Op0C))) {
+ Constant *NewRHS = ConstantExpr::get(I.getOpcode(),
+ cast<Constant>(Op0BO->getOperand(0)), Op1);
+
+ Value *NewShift = Builder.CreateShl(Op0BO->getOperand(1), Op1);
+ NewShift->takeName(Op0BO);
+
+ return BinaryOperator::CreateSub(NewRHS, NewShift);
+ }
}
}