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Fix a ValueTracking rule: RHS means operand 1, not 0. Add a simple

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ashr instcombine to help expose this code. And apply the fix to
SelectionDAG's copy of this code too.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65364 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman
2009-02-24 02:00:40 +00:00
parent c34fea3935
commit 0001e56f15
4 changed files with 16 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
lib/Analysis/ValueTracking.cpp
View File

@ -625,7 +625,7 @@ unsigned llvm::ComputeNumSignBits(Value *V, TargetData *TD, unsigned Depth) {
if (Tmp == 1) return 1; // Early out.
// Special case decrementing a value (ADD X, -1):
if (ConstantInt *CRHS = dyn_cast<ConstantInt>(U->getOperand(0)))
if (ConstantInt *CRHS = dyn_cast<ConstantInt>(U->getOperand(1)))
if (CRHS->isAllOnesValue()) {
APInt KnownZero(TyBits, 0), KnownOne(TyBits, 0);
APInt Mask = APInt::getAllOnesValue(TyBits);

2
lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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@ -2008,7 +2008,7 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, unsigned Depth) const{
if (Tmp == 1) return 1; // Early out.
// Special case decrementing a value (ADD X, -1):
if (ConstantSDNode *CRHS = dyn_cast<ConstantSDNode>(Op.getOperand(0)))
if (ConstantSDNode *CRHS = dyn_cast<ConstantSDNode>(Op.getOperand(1)))
if (CRHS->isAllOnesValue()) {
APInt KnownZero, KnownOne;
APInt Mask = APInt::getAllOnesValue(VTBits);

7
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -7031,7 +7031,12 @@ Instruction *InstCombiner::visitAShr(BinaryOperator &I) {
MaskedValueIsZero(Op0,
APInt::getSignBit(I.getType()->getPrimitiveSizeInBits())))
return BinaryOperator::CreateLShr(Op0, I.getOperand(1));
// Arithmetic shifting an all-sign-bit value is a no-op.
unsigned NumSignBits = ComputeNumSignBits(Op0);
if (NumSignBits == Op0->getType()->getPrimitiveSizeInBits())
return ReplaceInstUsesWith(I, Op0);
return 0;
}

8
test/Transforms/InstCombine/ashr-nop.ll Normal file
View File

@ -0,0 +1,8 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | not grep ashr
define i32 @foo(i32 %x) {
%o = and i32 %x, 1
%n = add i32 %o, -1
%t = ashr i32 %n, 17
ret i32 %t
}