[llvm-commits] CVS: llvm/lib/Transforms/Scalar/InstructionCombining.cpp

[Chris Lattner]

Changes in directory llvm/lib/Transforms/Scalar:

InstructionCombining.cpp updated: 1.506 -> 1.507
---
Log message:

Implement Transforms/InstCombine/shift-sra.ll:test0


---
Diffs of the changes:  (+20 -0)

 InstructionCombining.cpp |   20 ++++++++++++++++++++
 1 files changed, 20 insertions(+)


Index: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
diff -u llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.506 llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.507
--- llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.506	Sun Sep 17 23:22:48 2006
+++ llvm/lib/Transforms/Scalar/InstructionCombining.cpp	Sun Sep 17 23:31:40 2006
@@ -1089,6 +1089,26 @@
     }
     break;
   case Instruction::Shr:
+    // If this is an arithmetic shift right and only the low-bit is set, we can
+    // always convert this into a logical shr, even if the shift amount is
+    // variable.  The low bit of the shift cannot be an input sign bit unless
+    // the shift amount is >= the size of the datatype, which is undefined.
+    if (DemandedMask == 1 && I->getType()->isSigned()) {
+      // Convert the input to unsigned.
+      Instruction *NewVal = new CastInst(I->getOperand(0), 
+                                         I->getType()->getUnsignedVersion(),
+                                         I->getOperand(0)->getName());
+      InsertNewInstBefore(NewVal, *I);
+      // Perform the unsigned shift right.
+      NewVal = new ShiftInst(Instruction::Shr, NewVal, I->getOperand(1),
+                             I->getName());
+      InsertNewInstBefore(NewVal, *I);
+      // Then cast that to the destination type.
+      NewVal = new CastInst(NewVal, I->getType(), I->getName());
+      InsertNewInstBefore(NewVal, *I);
+      return UpdateValueUsesWith(I, NewVal);
+    }    
+    
     if (ConstantUInt *SA = dyn_cast<ConstantUInt>(I->getOperand(1))) {
       unsigned ShAmt = SA->getValue();