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Remove threading of Xor over selects and phis, with an explanation

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of why such threading is pointless.

llvm-svn: 119798
This commit is contained in:
Duncan Sands 2010-11-19 09:20:39 +00:00
parent f0b203d4a4
commit 56aefef080
1 changed files with 18 additions and 13 deletions
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31
lib/Analysis/InstructionSimplify.cpp
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@ -250,6 +250,16 @@ Value *llvm::SimplifyAddInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
}
// FIXME: Could pull several more out of instcombine.
// Threading Add over selects and phi nodes is pointless, so don't bother.
// Threading over the select in "A + select(cond, B, C)" means evaluating
// "A+B" and "A+C" and seeing if they are equal; but they are equal if and
// only if B and C are equal. If B and C are equal then (since we assume
// that operands have already been simplified) "select(cond, B, C)" should
// have been simplified to the common value of B and C already. Analysing
// "A+B" and "A+C" thus gains nothing, but costs compile time. Similarly
// for threading over phi nodes.
return 0;
}
@ -454,19 +464,14 @@ static Value *SimplifyXorInst(Value *Op0, Value *Op1, const TargetData *TD,
(A == Op0 || B == Op0))
return A == Op0 ? B : A;
// If the operation is with the result of a select instruction, check whether
// operating on either branch of the select always yields the same value.
if (MaxRecurse && (isa<SelectInst>(Op0) || isa<SelectInst>(Op1)))
if (Value *V = ThreadBinOpOverSelect(Instruction::Xor, Op0, Op1, TD, DT,
MaxRecurse-1))
return V;
// If the operation is with the result of a phi instruction, check whether
// operating on all incoming values of the phi always yields the same value.
if (MaxRecurse && (isa<PHINode>(Op0) || isa<PHINode>(Op1)))
if (Value *V = ThreadBinOpOverPHI(Instruction::Xor, Op0, Op1, TD, DT,
MaxRecurse-1))
return V;
// Threading Xor over selects and phi nodes is pointless, so don't bother.
// Threading over the select in "A ^ select(cond, B, C)" means evaluating
// "A^B" and "A^C" and seeing if they are equal; but they are equal if and
// only if B and C are equal. If B and C are equal then (since we assume
// that operands have already been simplified) "select(cond, B, C)" should
// have been simplified to the common value of B and C already. Analysing
// "A^B" and "A^C" thus gains nothing, but costs compile time. Similarly
// for threading over phi nodes.
return 0;
}