Add a new WeakVH value handle; NFC

This relands r301425.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301813 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/ValueHandle.h

@@ -34,7 +34,7 @@ protected:
   ///
   /// This is to avoid having a vtable for the light-weight handle pointers. The
   /// fully general Callback version does have a vtable.
-  enum HandleBaseKind { Assert, Callback, WeakTracking };
+  enum HandleBaseKind { Assert, Callback, Weak, WeakTracking };
 
   ValueHandleBase(const ValueHandleBase &RHS)
       : ValueHandleBase(RHS.PrevPair.getInt(), RHS) {}
@@ -46,7 +46,7 @@ protected:
   }
 
 private:
-  PointerIntPair<ValueHandleBase**, 2, HandleBaseKind> PrevPair;
+  PointerIntPair<ValueHandleBase**, 3, HandleBaseKind> PrevPair;
   ValueHandleBase *Next;
 
   Value *Val;
@@ -134,6 +134,42 @@ private:
   void AddToUseList();
 };
 
+/// \brief A nullable Value handle that is nullable.
+///
+/// This is a value handle that points to a value, and nulls itself
+/// out if that value is deleted.
+class WeakVH : public ValueHandleBase {
+public:
+  WeakVH() : ValueHandleBase(Weak) {}
+  WeakVH(Value *P) : ValueHandleBase(Weak, P) {}
+  WeakVH(const WeakVH &RHS)
+      : ValueHandleBase(Weak, RHS) {}
+
+  WeakVH &operator=(const WeakVH &RHS) = default;
+
+  Value *operator=(Value *RHS) {
+    return ValueHandleBase::operator=(RHS);
+  }
+  Value *operator=(const ValueHandleBase &RHS) {
+    return ValueHandleBase::operator=(RHS);
+  }
+
+  operator Value*() const {
+    return getValPtr();
+  }
+};
+
+// Specialize simplify_type to allow WeakVH to participate in
+// dyn_cast, isa, etc.
+template <> struct simplify_type<WeakVH> {
+  typedef Value *SimpleType;
+  static SimpleType getSimplifiedValue(WeakVH &WVH) { return WVH; }
+};
+template <> struct simplify_type<const WeakVH> {
+  typedef Value *SimpleType;
+  static SimpleType getSimplifiedValue(const WeakVH &WVH) { return WVH; }
+};
+
 /// \brief Value handle that is nullable, but tries to track the Value.
 ///
 /// This is a value handle that tries hard to point to a Value, even across

lib/IR/Value.cpp

@@ -820,8 +820,10 @@ void ValueHandleBase::ValueIsDeleted(Value *V) {
     switch (Entry->getKind()) {
     case Assert:
       break;
+    case Weak:
     case WeakTracking:
-      // WeakTracking just goes to null, which will unlink it from the list.
+      // WeakTracking and Weak just go to null, which unlinks them
+      // from the list.
       Entry->operator=(nullptr);
       break;
     case Callback:
@@ -869,7 +871,8 @@ void ValueHandleBase::ValueIsRAUWd(Value *Old, Value *New) {
 
     switch (Entry->getKind()) {
     case Assert:
-      // Asserting handle does not follow RAUW implicitly.
+    case Weak:
+      // Asserting and Weak handles do not follow RAUW implicitly.
       break;
     case WeakTracking:
       // Weak goes to the new value, which will unlink it from Old's list.

unittests/IR/ValueHandleTest.cpp

@@ -34,6 +34,24 @@ public:
   ConcreteCallbackVH(Value *V) : CallbackVH(V) {}
 };
 
+TEST_F(ValueHandle, WeakVH_BasicOperation) {
+  WeakVH WVH(BitcastV.get());
+  EXPECT_EQ(BitcastV.get(), WVH);
+  WVH = ConstantV;
+  EXPECT_EQ(ConstantV, WVH);
+
+  // Make sure I can call a method on the underlying Value.  It
+  // doesn't matter which method.
+  EXPECT_EQ(Type::getInt32Ty(Context), WVH->getType());
+  EXPECT_EQ(Type::getInt32Ty(Context), (*WVH).getType());
+
+  WVH = BitcastV.get();
+  BitcastV->replaceAllUsesWith(ConstantV);
+  EXPECT_EQ(WVH, BitcastV.get());
+  BitcastV.reset();
+  EXPECT_EQ(WVH, nullptr);
+}
+
 TEST_F(ValueHandle, WeakTrackingVH_BasicOperation) {
   WeakTrackingVH WVH(BitcastV.get());
   EXPECT_EQ(BitcastV.get(), WVH);