[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

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 5f21666f65a..098079abdf7 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -510,6 +510,20 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, Constant *Op1,
                                         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);
+      }
     }
   }