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

[Chris Lattner]

Changes in directory llvm/lib/Transforms/Scalar:

InstructionCombining.cpp updated: 1.341 -> 1.342
---
Log message:

Strength reduce SAR into SHR if there is no way sign bits could be shifted
in.  This tends to get cases like this:

  X = cast ubyte to int
  Y = shr int X, ...

Tested by: shift.ll:test24


---
Diffs of the changes:  (+10 -0)

 InstructionCombining.cpp |   10 ++++++++++
 1 files changed, 10 insertions(+)


Index: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
diff -u llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.341 llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.342
--- llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.341	Sat May  7 18:49:08 2005
+++ llvm/lib/Transforms/Scalar/InstructionCombining.cpp	Sun May  8 12:34:56 2005
@@ -3136,6 +3136,16 @@
       if (Instruction *R = FoldOpIntoSelect(I, SI, this))
         return R;
 
+  // See if we can turn a signed shr into an unsigned shr.
+  if (!isLeftShift && I.getType()->isSigned()) {
+    if (MaskedValueIsZero(Op0, ConstantInt::getMinValue(I.getType()))) {
+      Value *V = InsertCastBefore(Op0, I.getType()->getUnsignedVersion(), I);
+      V = InsertNewInstBefore(new ShiftInst(Instruction::Shr, V, Op1,
+                                            I.getName()), I);
+      return new CastInst(V, I.getType());
+    }
+  }
+  
   if (ConstantUInt *CUI = dyn_cast<ConstantUInt>(Op1)) {
     // shl uint X, 32 = 0 and shr ubyte Y, 9 = 0, ... just don't eliminate shr
     // of a signed value.