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Don't do: "(X & 4) >> 1 == 2 --> (X & 4) == 4" if there are more than one uses of the shift result.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@50118 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng
2008-04-23 00:38:06 +00:00
parent a5ddb59a13
commit f30752cbcc
2 changed files with 32 additions and 2 deletions
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5
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@@ -6065,13 +6065,14 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
// Otherwise, check to see if the bits shifted out are known to be zero. // Otherwise, check to see if the bits shifted out are known to be zero.
// If so, we can compare against the unshifted value: // If so, we can compare against the unshifted value:
// (X & 4) >> 1 == 2 --> (X & 4) == 4. // (X & 4) >> 1 == 2 --> (X & 4) == 4.
if (MaskedValueIsZero(LHSI->getOperand(0), if (LHSI->hasOneUse() &&
MaskedValueIsZero(LHSI->getOperand(0),
APInt::getLowBitsSet(Comp.getBitWidth(), ShAmtVal))) { APInt::getLowBitsSet(Comp.getBitWidth(), ShAmtVal))) {
return new ICmpInst(ICI.getPredicate(), LHSI->getOperand(0), return new ICmpInst(ICI.getPredicate(), LHSI->getOperand(0),
ConstantExpr::getShl(RHS, ShAmt)); ConstantExpr::getShl(RHS, ShAmt));
} }
if (LHSI->hasOneUse() || RHSV == 0) { if (LHSI->hasOneUse()) {
// Otherwise strength reduce the shift into an and. // Otherwise strength reduce the shift into an and.
APInt Val(APInt::getHighBitsSet(TypeBits, TypeBits - ShAmtVal)); APInt Val(APInt::getHighBitsSet(TypeBits, TypeBits - ShAmtVal));
Constant *Mask = ConstantInt::get(Val); Constant *Mask = ConstantInt::get(Val);

29
test/Transforms/InstCombine/shl-icmp.ll Normal file
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@@ -0,0 +1,29 @@
; RUN: llvm-as < %s | opt -instcombine -stats -disable-output |& \
; RUN: grep {Number of insts combined} | grep 5
define i8 @t1(i8 zeroext %x, i8 zeroext %y) zeroext nounwind {
entry:
%tmp1 = lshr i8 %x, 7
%cond1 = icmp ne i8 %tmp1, 0
br i1 %cond1, label %bb1, label %bb2
bb1:
ret i8 %tmp1
bb2:
%tmp2 = add i8 %tmp1, %y
ret i8 %tmp2
}
define i8 @t2(i8 zeroext %x) zeroext nounwind {
entry:
%tmp1 = lshr i8 %x, 7
%cond1 = icmp ne i8 %tmp1, 0
br i1 %cond1, label %bb1, label %bb2
bb1:
ret i8 0
bb2:
ret i8 1
}