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[InstSimplify] SimplifyICmpInst - icmp eq/ne %X, undef -> undef

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As discussed on PR41125 and D59363, we have a mismatch between icmp eq/ne cases with an undef operand:

When the other operand is constant we fold to undef (handled in ConstantFoldCompareInstruction)
When the other operand is non-constant we fold to a bool constant based on isTrueWhenEqual (handled in SimplifyICmpInst).

Neither is really wrong, but this patch changes the logic in SimplifyICmpInst to consistently fold to undef.

The NewGVN test change is annoying (as with most heavily reduced tests) but AFAICT I have kept the purpose of the test based on rL291968.

Differential Revision: https://reviews.llvm.org/D59541

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356456 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Simon Pilgrim
2019-03-19 14:08:23 +00:00
parent 92078fc49a
commit 07bbe5e292
3 changed files with 20 additions and 10 deletions
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lib/Analysis/InstructionSimplify.cpp
+7
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@@ -3045,9 +3045,16 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
std::swap(LHS, RHS);
Pred = CmpInst::getSwappedPredicate(Pred);
}
assert(!isa<UndefValue>(LHS) && "Unexpected icmp undef,%X");
Type *ITy = GetCompareTy(LHS); // The return type.
// For EQ and NE, we can always pick a value for the undef to make the
// predicate pass or fail, so we can return undef.
// Matches behavior in llvm::ConstantFoldCompareInstruction.
if (isa<UndefValue>(RHS) && ICmpInst::isEquality(Pred))
return UndefValue::get(ITy);
// icmp X, X -> true/false
// icmp X, undef -> true/false because undef could be X.
if (LHS == RHS || isa<UndefValue>(RHS))