From a09e18fcfab0e998526724357f8fc5ef7f4c3e7a Mon Sep 17 00:00:00 2001 From: Shuxin Yang Date: Tue, 4 Dec 2012 22:15:32 +0000 Subject: For rdar://12329730, last piece. This change attempts to simplify (X^Y) -> X or Y in the user's context if we know that only bits from X or Y are demanded. A minimized case is provided bellow. This change will simplify "t>>16" into "var1 >>16". ============================================================= unsigned foo (unsigned val1, unsigned val2) { unsigned t = val1 ^ 1234; return (t >> 16) | t; // NOTE: t is used more than once. } ============================================================= Note that if the "t" were used only once, the expression would be finally optimized as well. However, with with this change, the optimization will take place earlier. Reviewed by Nadav, Thanks a lot! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169317 91177308-0d34-0410-b5e6-96231b3b80d8 --- .../InstCombine/InstCombineSimplifyDemanded.cpp | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) (limited to 'lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp') diff --git a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp index c832ca5644..08aedb3200 100644 --- a/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp +++ b/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp @@ -200,8 +200,21 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask, if ((DemandedMask & (~LHSKnownZero) & RHSKnownOne) == (DemandedMask & (~LHSKnownZero))) return I->getOperand(1); + } else if (I->getOpcode() == Instruction::Xor) { + // We can simplify (X^Y) -> X or Y in the user's context if we know that + // only bits from X or Y are demanded. + + ComputeMaskedBits(I->getOperand(1), RHSKnownZero, RHSKnownOne, Depth+1); + ComputeMaskedBits(I->getOperand(0), LHSKnownZero, LHSKnownOne, Depth+1); + + // If all of the demanded bits are known zero on one side, return the + // other. + if ((DemandedMask & RHSKnownZero) == DemandedMask) + return I->getOperand(0); + if ((DemandedMask & LHSKnownZero) == DemandedMask) + return I->getOperand(1); } - + // Compute the KnownZero/KnownOne bits to simplify things downstream. ComputeMaskedBits(I, KnownZero, KnownOne, Depth); return 0; -- cgit v1.2.3-70-g09d2