Index: llvm/lib/Transforms/IPO/Attributor.cpp
===================================================================
--- llvm/lib/Transforms/IPO/Attributor.cpp
+++ llvm/lib/Transforms/IPO/Attributor.cpp
@@ -38,6 +38,8 @@
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Local.h"
 
+#include <unordered_set>
+
 #include <cassert>
 
 using namespace llvm;
@@ -1968,21 +1970,36 @@
 struct AAUndefinedBehaviorImpl : public AAUndefinedBehavior {
   AAUndefinedBehaviorImpl(const IRPosition &IRP) : AAUndefinedBehavior(IRP) {}
 
+  std::unordered_set<Instruction*> noUBLoads;
+
   /// See AbstractAttribute::initialize(...).
   void initialize(Attributor &A) override {
     AAUndefinedBehavior::initialize(A);
-
-    auto HandleLoadInstruction = [&](Instruction &I) {
-      // TODO: Do something for every load instruction...
-      return true;
-    };
-    A.checkForAllInstructions(HandleLoadInstruction, *this,
-                              {Instruction::Load});
   }
 
   /// See AbstractAttribute::updateImpl(...).
   ChangeStatus updateImpl(Attributor &A) override {
-    return indicatePessimisticFixpoint();
+    size_t prevSize = noUBLoads.size();
+
+    auto CantCauseUB = [&](Instruction &Iref) {
+      Instruction *I = &Iref;
+      // 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 or undef.
+        bool canCauseUB;
+        if (!canCauseUB) {
+          noUBLoads.insert(I);
+        }
+      }
+      return true;
+    };
+    A.checkForAllInstructions(CantCauseUB, *this, {Instruction::Load});
+    if (prevSize != noUBLoads.size()) {
+      return ChangeStatus::CHANGED;
+    }
+    return ChangeStatus::UNCHANGED;
   }
 
   /// See AbstractAttribute::getAsStr()