[ValueTracking] Add MustPreserveNullness arg to functions analyzing calls. (NFC)

[fhahn]

Summary

Some uses of getArgumentAliasingToReturnedPointer and
isIntrinsicReturningPointerAliasingArgumentWithoutCapturing require the
calls/intrinsics to preserve the nullness of the argument.

For alias analysis, the nullness property does not really come into
play.

This patch explicitly sets it to true. In D61669, the alias analysis
uses will be switched to not require preserving nullness.

[jdoerfert]

LGTM. One could have uses a default value (true) but making it explicit is fine with me as well.

llvm/trunk/include/llvm/Analysis/ValueTracking.h

class Value;
   bool getConstantStringInfo(const Value *V, StringRef &Str,
                              uint64_t Offset = 0, bool TrimAtNul = true);
 
   /// If we can compute the length of the string pointed to by the specified
   /// pointer, return 'len+1'.  If we can't, return 0.
   uint64_t GetStringLength(const Value *V, unsigned CharSize = 8);
 
   /// This function returns call pointer argument that is considered the same by
-  /// aliasing rules. You CAN'T use it to replace one value with another.
-  const Value *getArgumentAliasingToReturnedPointer(const CallBase *Call);
-  inline Value *getArgumentAliasingToReturnedPointer(CallBase *Call) {
+  /// aliasing rules. You CAN'T use it to replace one value with another. If
+  /// \p MustPreserveNullness is true, the call must preserve the nullness of
+  /// the pointer.
+  const Value *getArgumentAliasingToReturnedPointer(const CallBase *Call,
+                                                    bool MustPreserveNullness);
+  inline Value *
+  getArgumentAliasingToReturnedPointer(CallBase *Call,
+                                       bool MustPreserveNullness) {
     return const_cast<Value *>(getArgumentAliasingToReturnedPointer(
-        const_cast<const CallBase *>(Call)));
+        const_cast<const CallBase *>(Call), MustPreserveNullness));
   }
 
-  // {launder,strip}.invariant.group returns pointer that aliases its argument,
-  // and it only captures pointer by returning it.
-  // These intrinsics are not marked as nocapture, because returning is
-  // considered as capture. The arguments are not marked as returned neither,
-  // because it would make it useless.
+  /// {launder,strip}.invariant.group returns pointer that aliases its argument,
+  /// and it only captures pointer by returning it.
+  /// These intrinsics are not marked as nocapture, because returning is
+  /// considered as capture. The arguments are not marked as returned neither,
+  /// because it would make it useless. If \p MustPreserveNullness is true,
+  /// the intrinsic must preserve the nullness of the pointer.
   bool isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(
-      const CallBase *Call);
+      const CallBase *Call, bool MustPreserveNullness);
 
   /// This method strips off any GEP address adjustments and pointer casts from
   /// the specified value, returning the original object being addressed. Note
   /// that the returned value has pointer type if the specified value does. If
   /// the MaxLookup value is non-zero, it limits the number of instructions to
   /// be stripped off.
   Value *GetUnderlyingObject(Value *V, const DataLayout &DL,
                              unsigned MaxLookup = 6);

llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp

if (!GEPOp) {
         // intrinsics like launder.invariant.group, that can't be expressed with
         // the attributes, but have properties like returning aliasing pointer.
         // Because some analysis may assume that nocaptured pointer is not
         // returned from some special intrinsic (because function would have to
         // be marked with returns attribute), it is crucial to use this function
         // because it should be in sync with CaptureTracking. Not using it may
         // cause weird miscompilations where 2 aliasing pointers are assumed to
         // noalias.
-        if (auto *RP = getArgumentAliasingToReturnedPointer(Call)) {
+        if (auto *RP = getArgumentAliasingToReturnedPointer(Call, true)) {
           V = RP;
           continue;
         }
       }
 
       // If it's not a GEP, hand it off to SimplifyInstruction to see if it
       // can come up with something. This matches what GetUnderlyingObject does.
       if (const Instruction *I = dyn_cast<Instruction>(V))

llvm/trunk/lib/Analysis/CaptureTracking.cpp

case Instruction::Invoke: {
           Call->getType()->isVoidTy())
         break;
 
       // The pointer is not captured if returned pointer is not captured.
       // NOTE: CaptureTracking users should not assume that only functions
       // marked with nocapture do not capture. This means that places like
       // GetUnderlyingObject in ValueTracking or DecomposeGEPExpression
       // in BasicAA also need to know about this property.
-      if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(Call)) {
+      if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(Call,
+                                                                      true)) {
         AddUses(Call);
         break;
       }
 
       // Volatile operations effectively capture the memory location that they
       // load and store to.
       if (auto *MI = dyn_cast<MemIntrinsic>(Call))
         if (MI->isVolatile())

llvm/trunk/lib/Analysis/Loads.cpp

if (const GCRelocateInst *RelocateInst = dyn_cast<GCRelocateInst>(V))
     return isDereferenceableAndAlignedPointer(
         RelocateInst->getDerivedPtr(), Align, Size, DL, CtxI, DT, Visited);
 
   if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V))
     return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Align, Size,
                                               DL, CtxI, DT, Visited);
 
   if (const auto *Call = dyn_cast<CallBase>(V))
-    if (auto *RP = getArgumentAliasingToReturnedPointer(Call))
+    if (auto *RP = getArgumentAliasingToReturnedPointer(Call, true))
       return isDereferenceableAndAlignedPointer(RP, Align, Size, DL, CtxI, DT,
                                                 Visited);
 
   // If we don't know, assume the worst.
   return false;
 }
 
 bool llvm::isDereferenceableAndAlignedPointer(const Value *V, unsigned Align,

llvm/trunk/lib/Analysis/ValueTracking.cpp

if (V->getType()->isPointerTy()) {
     // A Load tagged with nonnull metadata is never null.
     if (const LoadInst *LI = dyn_cast<LoadInst>(V))
       if (Q.IIQ.getMetadata(LI, LLVMContext::MD_nonnull))
         return true;
 
     if (const auto *Call = dyn_cast<CallBase>(V)) {
       if (Call->isReturnNonNull())
         return true;
-      if (const auto *RP = getArgumentAliasingToReturnedPointer(Call))
+      if (const auto *RP = getArgumentAliasingToReturnedPointer(Call, true))
         return isKnownNonZero(RP, Depth, Q);
     }
   }
 
 
   // Check for recursive pointer simplifications.
   if (V->getType()->isPointerTy()) {
     if (isKnownNonNullFromDominatingCondition(V, Q.CxtI, Q.DT))
uint64_t llvm::GetStringLength(const Value *V, unsigned CharSize) {
 
   SmallPtrSet<const PHINode*, 32> PHIs;
   uint64_t Len = GetStringLengthH(V, PHIs, CharSize);
   // If Len is ~0ULL, we had an infinite phi cycle: this is dead code, so return
   // an empty string as a length.
   return Len == ~0ULL ? 1 : Len;
 }
 
-const Value *llvm::getArgumentAliasingToReturnedPointer(const CallBase *Call) {
+const Value *
+llvm::getArgumentAliasingToReturnedPointer(const CallBase *Call,
+                                           bool MustPreserveNullness) {
   assert(Call &&
          "getArgumentAliasingToReturnedPointer only works on nonnull calls");
   if (const Value *RV = Call->getReturnedArgOperand())
     return RV;
   // This can be used only as a aliasing property.
-  if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(Call))
+  if (isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(
+          Call, MustPreserveNullness))
     return Call->getArgOperand(0);
   return nullptr;
 }
 
 bool llvm::isIntrinsicReturningPointerAliasingArgumentWithoutCapturing(
-    const CallBase *Call) {
+    const CallBase *Call, bool MustPreserveNullness) {
   return Call->getIntrinsicID() == Intrinsic::launder_invariant_group ||
          Call->getIntrinsicID() == Intrinsic::strip_invariant_group ||
          Call->getIntrinsicID() == Intrinsic::aarch64_irg ||
          Call->getIntrinsicID() == Intrinsic::aarch64_tagp;
 }
 
 /// \p PN defines a loop-variant pointer to an object.  Check if the
 /// previous iteration of the loop was referring to the same object as \p PN.
if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
         // intrinsics like launder.invariant.group, that can't be expressed with
         // the attributes, but have properties like returning aliasing pointer.
         // Because some analysis may assume that nocaptured pointer is not
         // returned from some special intrinsic (because function would have to
         // be marked with returns attribute), it is crucial to use this function
         // because it should be in sync with CaptureTracking. Not using it may
         // cause weird miscompilations where 2 aliasing pointers are assumed to
         // noalias.
-        if (auto *RP = getArgumentAliasingToReturnedPointer(Call)) {
+        if (auto *RP = getArgumentAliasingToReturnedPointer(Call, true)) {
           V = RP;
           continue;
         }
       }
 
       // See if InstructionSimplify knows any relevant tricks.
       if (Instruction *I = dyn_cast<Instruction>(V))
         // TODO: Acquire a DominatorTree and AssumptionCache and use them.