Index: llvm/include/llvm/Transforms/IPO/Attributor.h
===================================================================
--- llvm/include/llvm/Transforms/IPO/Attributor.h
+++ llvm/include/llvm/Transforms/IPO/Attributor.h
@@ -1686,6 +1686,29 @@
   static const char ID;
 };
 
+/// An abstract attribute for undefined behavior.
+struct AAUndefinedBehavior
+    : public StateWrapper<BooleanState, AbstractAttribute>,
+      public IRPosition {
+  AAUndefinedBehavior(const IRPosition &IRP) : IRPosition(IRP) {}
+
+  /// Return true if "undefined behavior" is assumed.
+  bool isAssumedToCauseUB() const { return getAssumed(); }
+
+  /// Return true if "undefined behavior" is known.
+  bool isKnownToCauseUB() const { return getKnown(); }
+
+  /// Return an IR position, see struct IRPosition.
+  const IRPosition &getIRPosition() const override { return *this; }
+
+  /// Create an abstract attribute view for the position \p IRP.
+  static AAUndefinedBehavior &createForPosition(const IRPosition &IRP,
+                                                Attributor &A);
+
+  /// Unique ID (due to the unique address)
+  static const char ID;
+};
+
 /// An abstract interface to determine reachability of point A to B.
 struct AAReachability : public StateWrapper<BooleanState, AbstractAttribute>,
                         public IRPosition {
Index: llvm/lib/Transforms/IPO/Attributor.cpp
===================================================================
--- llvm/lib/Transforms/IPO/Attributor.cpp
+++ llvm/lib/Transforms/IPO/Attributor.cpp
@@ -1987,6 +1987,73 @@
   void trackStatistics() const override { STATS_DECLTRACK_CS_ATTR(norecurse); }
 };
 
+/// -------------------- Undefined-Behavior Attributes ------------------------
+
+struct AAUndefinedBehaviorImpl : public AAUndefinedBehavior {
+  AAUndefinedBehaviorImpl(const IRPosition &IRP) : AAUndefinedBehavior(IRP) {}
+
+  SmallPtrSet<Instruction *, 8> NoUBLoads;
+
+  /// See AbstractAttribute::initialize(...).
+  void initialize(Attributor &A) override {
+    AAUndefinedBehavior::initialize(A);
+  }
+
+  /// See AbstractAttribute::updateImpl(...).
+  ChangeStatus updateImpl(Attributor &A) override {
+    size_t PrevSize = NoUBLoads.size();
+
+    auto CannotCauseUB = [&](Instruction &I) {
+      // If this load instruction is not one of the instructions
+      // that are already assumed to cause UB.
+      if (NoUBLoads.find(&I) == NoUBLoads.end()) {
+        // Find whether it can cause UB by checking if the pointer
+        // is null. However, null dereference is defined for some
+        // targets, so it is assumed to cause UB only for those
+        // that it is not.
+        bool CanCauseUB = false;
+        Value *PtrOp = cast<LoadInst>(&I)->getPointerOperand();
+        if (isa<ConstantPointerNull>(PtrOp)) {
+          Type *PtrTy = PtrOp->getType();
+          const Function *F = I.getFunction();
+          bool NullPointerIsDefined =
+              F ? llvm::NullPointerIsDefined(F, PtrTy->getPointerAddressSpace())
+                : true;
+          if (!NullPointerIsDefined) {
+            CanCauseUB = true;
+          }
+        }
+        if (!CanCauseUB) {
+          NoUBLoads.insert(&I);
+        }
+      }
+      return true;
+    };
+    A.checkForAllInstructions(CannotCauseUB, *this, {Instruction::Load});
+    if (PrevSize != NoUBLoads.size())
+      return ChangeStatus::CHANGED;
+    return ChangeStatus::UNCHANGED;
+  }
+
+  /// See AbstractAttribute::getAsStr()
+  const std::string getAsStr() const override {
+    return getAssumed() ? "undefined-behavior" : "no-ub";
+  }
+};
+
+struct AAUndefinedBehaviorFunction final : AAUndefinedBehaviorImpl {
+  AAUndefinedBehaviorFunction(const IRPosition &IRP)
+      : AAUndefinedBehaviorImpl(IRP) {}
+
+  /// See AbstractAttribute::trackStatistics()
+  void trackStatistics() const override {
+    STATS_DECL(UndefinedBehaviorInstruction, Instruction,
+               "Number of instructions known to have UB");
+    BUILD_STAT_NAME(UndefinedBehaviorInstruction, Instruction) +=
+        NoUBLoads.size();
+  }
+};
+
 /// ------------------------ Will-Return Attributes ----------------------------
 
 // Helper function that checks whether a function has any cycle.
@@ -5523,6 +5590,9 @@
   // Every function might be "will-return".
   getOrCreateAAFor<AAWillReturn>(FPos);
 
+  // Every function might contain instructions that cause "undefined behavior".
+  getOrCreateAAFor<AAUndefinedBehavior>(FPos);
+
   // Every function can be nounwind.
   getOrCreateAAFor<AANoUnwind>(FPos);
 
@@ -5827,6 +5897,7 @@
 const char AANonNull::ID = 0;
 const char AANoRecurse::ID = 0;
 const char AAWillReturn::ID = 0;
+const char AAUndefinedBehavior::ID = 0;
 const char AANoAlias::ID = 0;
 const char AAReachability::ID = 0;
 const char AANoReturn::ID = 0;
@@ -5949,6 +6020,7 @@
 
 CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAHeapToStack)
 CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAReachability)
+CREATE_FUNCTION_ONLY_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAUndefinedBehavior)
 
 CREATE_NON_RET_ABSTRACT_ATTRIBUTE_FOR_POSITION(AAMemoryBehavior)