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

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

llvm-svn: 356456

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 1c839fed171..851306bfe20 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -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))