From 147a8edb1b5b0a1b09591d4b09fa43ece7b37749 Mon Sep 17 00:00:00 2001
From: Sanjoy Das <sanjoy@playingwithpointers.com>
Date: Wed, 26 Apr 2017 16:20:59 +0000
Subject: [PATCH] Add a new WeakVH value handle; NFC

Summary:
WeakVH nulls itself out if the value it was tracking gets deleted, but
it does not track RAUW.

Reviewers: dblaikie, davide

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D32267

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301425 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/llvm/IR/ValueHandle.h    | 40 ++++++++++++++++++++++++++++++--
 lib/IR/Value.cpp                 |  7 ++++--
 unittests/IR/ValueHandleTest.cpp | 18 ++++++++++++++
 3 files changed, 61 insertions(+), 4 deletions(-)

diff --git a/include/llvm/IR/ValueHandle.h b/include/llvm/IR/ValueHandle.h
index 7109c7b19f4..cba06671314 100644
--- a/include/llvm/IR/ValueHandle.h
+++ b/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, Tracking, WeakTracking };
+  enum HandleBaseKind { Assert, Callback, Tracking, 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* V;
@@ -126,6 +126,42 @@ 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
diff --git a/lib/IR/Value.cpp b/lib/IR/Value.cpp
index 23a0f7a2601..18e44dcd352 100644
--- a/lib/IR/Value.cpp
+++ b/lib/IR/Value.cpp
@@ -825,8 +825,10 @@ void ValueHandleBase::ValueIsDeleted(Value *V) {
       // pointer.
       Entry->operator=(DenseMapInfo<Value *>::getTombstoneKey());
       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:
@@ -874,7 +876,8 @@ 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 Tracking:
       // Tracking goes to new value like a WeakTrackingVH. Note that this may
diff --git a/unittests/IR/ValueHandleTest.cpp b/unittests/IR/ValueHandleTest.cpp
index f4cacd3f7dd..191fdbb8f0f 100644
--- a/unittests/IR/ValueHandleTest.cpp
+++ b/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);
-- 
2.50.1