[ValueTracking] Add isGuaranteedNotToBeUndef() variant (NFC)

We have a bunch of places where we have to guard against undef
to avoid multi-use issues, but would be fine with poison. Use a
different function for these to make it clear, and to indicate that
this check can be removed once we no longer support undef. I've
replaced some of the obvious cases, but there's probably more.

For now, the implementation is the same as UndefOrPoison, it just
has a more precise name.

Author: nikic

llvm/include/llvm/Analysis/ValueTracking.h

@@ -1002,11 +1002,19 @@ bool isGuaranteedNotToBeUndefOrPoison(const Value *V,
                                       const Instruction *CtxI = nullptr,
                                       const DominatorTree *DT = nullptr,
                                       unsigned Depth = 0);
+
+/// Returns true if V cannot be poison, but may be undef.
 bool isGuaranteedNotToBePoison(const Value *V, AssumptionCache *AC = nullptr,
                                const Instruction *CtxI = nullptr,
                                const DominatorTree *DT = nullptr,
                                unsigned Depth = 0);
 
+/// Returns true if V cannot be undef, but may be poison.
+bool isGuaranteedNotToBeUndef(const Value *V, AssumptionCache *AC = nullptr,
+                              const Instruction *CtxI = nullptr,
+                              const DominatorTree *DT = nullptr,
+                              unsigned Depth = 0);
+
 /// Return true if undefined behavior would provable be executed on the path to
 /// OnPathTo if Root produced a posion result.  Note that this doesn't say
 /// anything about whether OnPathTo is actually executed or whether Root is

llvm/lib/Analysis/LazyValueInfo.cpp

@@ -880,7 +880,7 @@ LazyValueInfoImpl::solveBlockValueSelect(SelectInst *SI, BasicBlock *BB) {
   Value *Cond = SI->getCondition();
   // If the value is undef, a different value may be chosen in
   // the select condition.
-  if (isGuaranteedNotToBeUndefOrPoison(Cond, AC)) {
+  if (isGuaranteedNotToBeUndef(Cond, AC)) {
     TrueVal = intersect(TrueVal,
                         getValueFromCondition(SI->getTrueValue(), Cond, true));
     FalseVal = intersect(
@@ -1719,7 +1719,7 @@ ConstantRange LazyValueInfo::getConstantRangeAtUse(const Use &U,
     if (auto *SI = dyn_cast<SelectInst>(CurrI)) {
       // If the value is undef, a different value may be chosen in
       // the select condition and at use.
-      if (!isGuaranteedNotToBeUndefOrPoison(SI->getCondition(), AC))
+      if (!isGuaranteedNotToBeUndef(SI->getCondition(), AC))
         break;
       if (CurrU->getOperandNo() == 1)
         CondVal = getValueFromCondition(V, SI->getCondition(), true);

llvm/lib/Analysis/ValueTracking.cpp

@@ -388,10 +388,9 @@ static void computeKnownBitsMul(const Value *Op0, const Value *Op1, bool NSW,
   }
 
   bool SelfMultiply = Op0 == Op1;
-  // TODO: SelfMultiply can be poison, but not undef.
   if (SelfMultiply)
     SelfMultiply &=
-        isGuaranteedNotToBeUndefOrPoison(Op0, Q.AC, Q.CxtI, Q.DT, Depth + 1);
+        isGuaranteedNotToBeUndef(Op0, Q.AC, Q.CxtI, Q.DT, Depth + 1);
   Known = KnownBits::mul(Known, Known2, SelfMultiply);
 
   // Only make use of no-wrap flags if we failed to compute the sign bit
@@ -6488,7 +6487,7 @@ OverflowResult llvm::computeOverflowForUnsignedSub(const Value *LHS,
   //       See simplifyICmpWithBinOpOnLHS() for candidates.
   if (match(RHS, m_URem(m_Specific(LHS), m_Value())) ||
       match(RHS, m_NUWSub(m_Specific(LHS), m_Value())))
-    if (isGuaranteedNotToBeUndefOrPoison(LHS, SQ.AC, SQ.CxtI, SQ.DT))
+    if (isGuaranteedNotToBeUndef(LHS, SQ.AC, SQ.CxtI, SQ.DT))
       return OverflowResult::NeverOverflows;
 
   // Checking for conditions implied by dominating conditions may be expensive.
@@ -6521,7 +6520,7 @@ OverflowResult llvm::computeOverflowForSignedSub(const Value *LHS,
   // then determining no-overflow may allow other transforms.
   if (match(RHS, m_SRem(m_Specific(LHS), m_Value())) ||
       match(RHS, m_NSWSub(m_Specific(LHS), m_Value())))
-    if (isGuaranteedNotToBeUndefOrPoison(LHS, SQ.AC, SQ.CxtI, SQ.DT))
+    if (isGuaranteedNotToBeUndef(LHS, SQ.AC, SQ.CxtI, SQ.DT))
       return OverflowResult::NeverOverflows;
 
   // If LHS and RHS each have at least two sign bits, the subtraction
@@ -6982,6 +6981,13 @@ bool llvm::isGuaranteedNotToBePoison(const Value *V, AssumptionCache *AC,
   return ::isGuaranteedNotToBeUndefOrPoison(V, AC, CtxI, DT, Depth, true);
 }
 
+bool llvm::isGuaranteedNotToBeUndef(const Value *V, AssumptionCache *AC,
+                                    const Instruction *CtxI,
+                                    const DominatorTree *DT, unsigned Depth) {
+  // TODO: This is currently equivalent to isGuaranteedNotToBeUndefOrPoison().
+  return ::isGuaranteedNotToBeUndefOrPoison(V, AC, CtxI, DT, Depth, false);
+}
+
 /// Return true if undefined behavior would provably be executed on the path to
 /// OnPathTo if Root produced a posion result.  Note that this doesn't say
 /// anything about whether OnPathTo is actually executed or whether Root is

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -757,7 +757,7 @@ static bool expandUDivOrURem(BinaryOperator *Instr, const ConstantRange &XCR,
     // NOTE: this transformation introduces two uses of X,
     //       but it may be undef so we must freeze it first.
     Value *FrozenX = X;
-    if (!isGuaranteedNotToBeUndefOrPoison(X))
+    if (!isGuaranteedNotToBeUndef(X))
       FrozenX = B.CreateFreeze(X, X->getName() + ".frozen");
     auto *AdjX = B.CreateNUWSub(FrozenX, Y, Instr->getName() + ".urem");
     auto *Cmp =