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Do not let MaskedValueIsZero consider undef to be zero, for reasons
explained in the comment. This fixes UnitTests/2003-09-18-BitFieldTest on darwin git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22483 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -1317,7 +1317,13 @@ struct FoldSetCCLogical {
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/// this predicate to simplify operations downstream. V and Mask are known to
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/// be the same type.
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static bool MaskedValueIsZero(Value *V, ConstantIntegral *Mask) {
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if (isa<UndefValue>(V) || Mask->isNullValue())
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// Note, we cannot consider 'undef' to be "IsZero" here. The problem is that
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// we cannot optimize based on the assumption that it is zero without changing
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// to to an explicit zero. If we don't change it to zero, other code could
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// optimized based on the contradictory assumption that it is non-zero.
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// Because instcombine aggressively folds operations with undef args anyway,
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// this won't lose us code quality.
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if (Mask->isNullValue())
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return true;
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if (ConstantIntegral *CI = dyn_cast<ConstantIntegral>(V))
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return ConstantExpr::getAnd(CI, Mask)->isNullValue();
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@ -3134,7 +3140,7 @@ Instruction *InstCombiner::visitShiftInst(ShiftInst &I) {
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return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
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}
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if (isa<UndefValue>(Op1)) {
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if (isLeftShift || I.getType()->isUnsigned())
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if (isLeftShift || I.getType()->isUnsigned())// X << undef, X >>u undef -> 0
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return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
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else
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return ReplaceInstUsesWith(I, Op0); // X >>s undef -> X
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