[InstCombine] (1 << (C - x)) -> ((1 << C) >> x) if C is bitwidth - 1

Summary:
```
%a = sub i32 31, %x
%r = shl i32 1, %a
  =>
%d = shl i32 1, 31
%r = lshr i32 %d, %x

Done: 1
Optimization is correct!
```

https://rise4fun.com/Alive/btZm

Reviewers: spatel, lebedev.ri, nikic

Reviewed By: lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63652

llvm-svn: 364073

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 21f1ff56602..655f40a0de0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -582,6 +582,8 @@ Instruction *InstCombiner::visitShl(BinaryOperator &I) {
 
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   Type *Ty = I.getType();
+  unsigned BitWidth = Ty->getScalarSizeInBits();
+
   const APInt *ShAmtAPInt;
   if (match(Op1, m_APInt(ShAmtAPInt))) {
     unsigned ShAmt = ShAmtAPInt->getZExtValue();
@@ -670,6 +672,12 @@ Instruction *InstCombiner::visitShl(BinaryOperator &I) {
       return BinaryOperator::CreateMul(X, ConstantExpr::getShl(C2, C1));
   }
 
+  // (1 << (C - x)) -> ((1 << C) >> x) if C is bitwidth - 1
+  if (match(Op0, m_One()) &&
+      match(Op1, m_Sub(m_SpecificInt(BitWidth - 1), m_Value(X))))
+    return BinaryOperator::CreateLShr(
+        ConstantInt::get(Ty, APInt::getSignMask(BitWidth)), X);
+
   return nullptr;
 }