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InstCombine: Add a combine that turns (2^n)-1 ^ x back into (2^n)-1 - x iff x is smaller than 2^n and it fuses with a following add.

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This was intended to undo the sub canonicalization in cases where it's not profitable, but it also
finds some cases on it's own.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147256 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2011-12-24 17:31:53 +00:00
parent 009da05e75
commit 49064ff770
2 changed files with 25 additions and 0 deletions
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13
lib/Transforms/InstCombine/InstCombineAddSub.cpp
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@ -136,6 +136,19 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
Value *NewShl = Builder->CreateShl(XorLHS, ShAmt, "sext");
return BinaryOperator::CreateAShr(NewShl, ShAmt);
}
// If this is a xor that was canonicalized from a sub, turn it back into
// a sub and fuse this add with it.
if (LHS->hasOneUse() && (XorRHS->getValue()+1).isPowerOf2()) {
IntegerType *IT = cast<IntegerType>(I.getType());
APInt Mask = APInt::getAllOnesValue(IT->getBitWidth());
APInt LHSKnownOne(IT->getBitWidth(), 0);
APInt LHSKnownZero(IT->getBitWidth(), 0);
ComputeMaskedBits(XorLHS, Mask, LHSKnownZero, LHSKnownOne);
if ((XorRHS->getValue() | LHSKnownZero).isAllOnesValue())
return BinaryOperator::CreateSub(ConstantExpr::getAdd(XorRHS, CI),
XorLHS);
}
}
}

12
test/Transforms/InstCombine/sub-xor.ll
View File

@ -23,3 +23,15 @@ define i32 @test2(i32 %x) nounwind {
; CHECK-NEXT: xor i32 %count, 31
; CHECK-NEXT: ret
}
define i32 @test3(i32 %x) nounwind {
%and = and i32 %x, 31
%sub = xor i32 31, %and
%add = add i32 %sub, 42
ret i32 %add
; CHECK: @test3
; CHECK-NEXT: and i32 %x, 31
; CHECK-NEXT: sub i32 73, %and
; CHECK-NEXT: ret
}