Bug 1443592: Part 1: Introduce opaque LiveSavedFrameCache::Key type for cache keys. r=fitzgen

By design, the LiveSavedFrameCache holds the addresses of both live and dead
stack frames. This change wraps those addresses in an opaque type that can only
be compared for equality with other such values, and provides no interface to
retrieve the underlying pointer, ensuring statically that we will not
accidentally use a cache key to access memory.

MozReview-Commit-ID: 9Wom5gFVQls

--HG--
extra : rebase_source : e3dff595085973f1f10f57a67c192656c0bcd866

js/src/vm/SavedStacks.cpp

@@ -110,14 +110,14 @@ LiveSavedFrameCache::find(JSContext* cx, FrameIter& frameIter, MutableHandleSave
     Maybe<FramePtr> maybeFramePtr = getFramePtr(frameIter);
     MOZ_ASSERT(maybeFramePtr.isSome());
 
-    FramePtr framePtr(*maybeFramePtr);
+    Key key(*maybeFramePtr);
     const jsbytecode* pc = frameIter.pc();
     size_t numberStillValid = 0;
 
     frame.set(nullptr);
     for (auto* p = frames->begin(); p < frames->end(); p++) {
         numberStillValid++;
-        if (framePtr == p->framePtr && pc == p->pc) {
+        if (key == p->key && pc == p->pc) {
             frame.set(p->savedFrame);
             break;
         }

js/src/vm/Stack.h

@@ -1173,14 +1173,27 @@ class LiveSavedFrameCache
     using FramePtr = mozilla::Variant<AbstractFramePtr, jit::CommonFrameLayout*>;
 
   private:
+    // A key in the cache: the address of a frame, live or dead, for which we
+    // can cache SavedFrames. Since the pointer may not be live, the only
+    // operation this type permits is comparison.
+    class Key {
+        FramePtr framePtr;
+
+      public:
+        MOZ_IMPLICIT Key(const FramePtr& framePtr) : framePtr(framePtr) { }
+
+        bool operator==(const Key& rhs) const { return rhs.framePtr == this->framePtr; }
+        bool operator!=(const Key& rhs) const { return !(rhs == *this); }
+    };
+
     struct Entry
     {
-        const FramePtr       framePtr;
+        const Key            key;
         const jsbytecode*    pc;
         HeapPtr<SavedFrame*> savedFrame;
 
-        Entry(const FramePtr& framePtr, const jsbytecode* pc, SavedFrame* savedFrame)
-          : framePtr(framePtr)
+        Entry(const Key& key, const jsbytecode* pc, SavedFrame* savedFrame)
+          : key(key)
           , pc(pc)
           , savedFrame(savedFrame)
         { }