Changes in directory llvm/lib/Transforms/Scalar:
InstructionCombining.cpp updated: 1.553 -> 1.554
---
Log message:
Teach instcombine to turn trunc(srl x, c) -> srl (trunc(x), c) when safe.
This implements InstCombine/cast.ll:test34. It fires hundreds of times on
176.gcc.
---
Diffs of the changes: (+33 -1)
InstructionCombining.cpp | 34 +++++++++++++++++++++++++++++++++-
1 files changed, 33 insertions(+), 1 deletion(-)
Index: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
diff -u llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.553
llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.554
--- llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.553 Tue Nov 28
23:02:16 2006
+++ llvm/lib/Transforms/Scalar/InstructionCombining.cpp Wed Nov 29 01:04:07 2006
@@ -5986,7 +5986,39 @@
}
Instruction *InstCombiner::visitTrunc(CastInst &CI) {
- return commonIntCastTransforms(CI);
+ if (Instruction *Result = commonIntCastTransforms(CI))
+ return Result;
+
+ Value *Src = CI.getOperand(0);
+ const Type *Ty = CI.getType();
+ unsigned DestBitWidth = Ty->getPrimitiveSizeInBits();
+
+ if (Instruction *SrcI = dyn_cast<Instruction>(Src)) {
+ switch (SrcI->getOpcode()) {
+ default: break;
+ case Instruction::LShr:
+ // We can shrink lshr to something smaller if we know the bits shifted in
+ // are already zeros.
+ if (ConstantInt *ShAmtV = dyn_cast<ConstantInt>(SrcI->getOperand(1))) {
+ unsigned ShAmt = ShAmtV->getZExtValue();
+
+ // Get a mask for the bits shifting in.
+ uint64_t Mask = (~0ULL >> (64-ShAmt)) << DestBitWidth;
+ if (SrcI->hasOneUse() && MaskedValueIsZero(SrcI->getOperand(0), Mask))
{
+ if (ShAmt >= DestBitWidth) // All zeros.
+ return ReplaceInstUsesWith(CI, Constant::getNullValue(Ty));
+
+ // Okay, we can shrink this. Truncate the input, then return a new
+ // shift.
+ Value *V = InsertCastBefore(SrcI->getOperand(0), Ty, CI);
+ return new ShiftInst(Instruction::LShr, V, SrcI->getOperand(1));
+ }
+ }
+ break;
+ }
+ }
+
+ return 0;
}
Instruction *InstCombiner::visitZExt(CastInst &CI) {
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