Steal a bit from the gen ID instead of managing two atomic values.

include/core/SkPixelRef.h

 /*
  * Copyright 2008 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkPixelRef_DEFINED
 #define SkPixelRef_DEFINED
 
@@ skipping 301 matching lines @@
 private:
     SkBaseMutex*    fMutex; // must remain in scope for the life of this object
 
     // mostly const. fInfo.fAlpahType can be changed at runtime.
     const SkImageInfo fInfo;
 
     // LockRec is only valid if we're in a locked state (isLocked())
     LockRec         fRec;
     int             fLockCount;
 
-    mutable SkAtomic<uint32_t> fGenerationID;
-    mutable SkAtomic<bool>     fUniqueGenerationID;
-    SkAtomic<bool>             fAddedToCache;
+    // Bottom bit indicates the Gen ID is unique.
+    bool genIDIsUnique() const { return SkToBool(fTaggedGenID.load() & 1); }
+    mutable SkAtomic<uint32_t> fTaggedGenID;
+
 #ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
     const uint32_t fStableID;
 #endif
 
     SkTDArray<GenIDChangeListener*> fGenIDChangeListeners;  // pointers are owned
 
     SkString    fURI;
 
+    // Set true by caches when they cache content that's derived from the current pixels.
+    SkAtomic<bool> fAddedToCache;
     // can go from false to true, but never from true to false
-    bool    fIsImmutable;
+    bool fIsImmutable;
     // only ever set in constructor, const after that
-    bool    fPreLocked;
+    bool fPreLocked;
 
     void needsNewGenID();
     void callGenIDChangeListeners();
 
     void setMutex(SkBaseMutex* mutex);
 
     // When copying a bitmap to another with the same shape and config, we can safely
     // clone the pixelref generation ID too, which makes them equivalent under caching.
     friend class SkBitmap;  // only for cloneGenID
     void cloneGenID(const SkPixelRef&);
 
     typedef SkRefCnt INHERITED;
 };
 
 class SkPixelRefFactory : public SkRefCnt {
 public:
     /**
      *  Allocate a new pixelref matching the specified ImageInfo, allocating
      *  the memory for the pixels. If the ImageInfo requires a ColorTable,
      *  the pixelref will ref() the colortable.
      *  On failure return NULL.
      */
     virtual SkPixelRef* create(const SkImageInfo&, size_t rowBytes, SkColorTable*) = 0;
 };
 
 #endif