cache private readback for gpu-images

Does not try to cache calls to readPixels at the moment:
- not triggered by drawing
- not clear if we want to perform any pixel transformations (that readPixels allows) on the GPU or CPU

Can consider that another time.

BUG=513695

Review URL: https://codereview.chromium.org/1262923003
diff --git a/src/core/SkBitmapCache.cpp b/src/core/SkBitmapCache.cpp
index aabf87a..3f1feac 100644
--- a/src/core/SkBitmapCache.cpp
+++ b/src/core/SkBitmapCache.cpp
@@ -142,6 +142,20 @@
     }
 }
 
+bool SkBitmapCache::Find(uint32_t genID, SkBitmap* result, SkResourceCache* localCache) {
+    BitmapKey key(genID, SK_Scalar1, SK_Scalar1, SkIRect::MakeEmpty());
+
+    return CHECK_LOCAL(localCache, find, Find, key, BitmapRec::Finder, result);
+}
+
+void SkBitmapCache::Add(uint32_t genID, const SkBitmap& result, SkResourceCache* localCache) {
+    SkASSERT(result.isImmutable());
+
+    BitmapRec* rec = SkNEW_ARGS(BitmapRec, (genID, 1, 1, SkIRect::MakeEmpty(), result));
+
+    CHECK_LOCAL(localCache, add, Add, rec);
+}
+
 //////////////////////////////////////////////////////////////////////////////////////////
 //////////////////////////////////////////////////////////////////////////////////////////
 
diff --git a/src/core/SkBitmapCache.h b/src/core/SkBitmapCache.h
index de50aab..c758942 100644
--- a/src/core/SkBitmapCache.h
+++ b/src/core/SkBitmapCache.h
@@ -52,6 +52,10 @@
      */
     static bool Add(SkPixelRef*, const SkIRect& subset, const SkBitmap& result,
                     SkResourceCache* localCache = NULL);
+
+    static bool Find(uint32_t genID, SkBitmap* result, SkResourceCache* localCache = NULL);
+    // todo: eliminate the need to specify ID, since it should == the bitmap's
+    static void Add(uint32_t genID, const SkBitmap&, SkResourceCache* localCache = NULL);
 };
 
 class SkMipMapCache {
diff --git a/src/core/SkPixelRef.cpp b/src/core/SkPixelRef.cpp
index e79dfba..2a46385 100644
--- a/src/core/SkPixelRef.cpp
+++ b/src/core/SkPixelRef.cpp
@@ -356,6 +356,18 @@
 void SkPixelRef::setImmutable() {
     fMutability = kImmutable;
 }
+
+void SkPixelRef::setImmutableWithID(uint32_t genID) {
+    /*
+     *  We are forcing the genID to match an external value. The caller must ensure that this
+     *  value does not conflict with other content.
+     *
+     *  One use is to force this pixelref's id to match an SkImage's id
+     */
+    fMutability = kImmutable;
+    fTaggedGenID.store(genID);
+}
+
 void SkPixelRef::setTemporarilyImmutable() {
     SkASSERT(fMutability != kImmutable);
     fMutability = kTemporarilyImmutable;
diff --git a/src/image/SkImage_Gpu.cpp b/src/image/SkImage_Gpu.cpp
index c89c08d..d410de8 100644
--- a/src/image/SkImage_Gpu.cpp
+++ b/src/image/SkImage_Gpu.cpp
@@ -5,13 +5,14 @@
  * found in the LICENSE file.
  */
 
+#include "SkBitmapCache.h"
 #include "SkImage_Gpu.h"
 #include "GrContext.h"
 #include "GrDrawContext.h"
 #include "effects/GrYUVtoRGBEffect.h"
 #include "SkCanvas.h"
 #include "SkGpuDevice.h"
-
+#include "SkPixelRef.h"
 
 SkImage_Gpu::SkImage_Gpu(int w, int h, uint32_t uniqueID, SkAlphaType at, GrTexture* tex,
                          int sampleCountForNewSurfaces, SkSurface::Budgeted budgeted)
@@ -22,6 +23,12 @@
     , fBudgeted(budgeted)
     {}
 
+SkImage_Gpu::~SkImage_Gpu() {
+    if (fAddedRasterVersionToCache.load()) {
+        SkNotifyBitmapGenIDIsStale(this->uniqueID());
+    }
+}
+
 SkSurface* SkImage_Gpu::onNewSurface(const SkImageInfo& info, const SkSurfaceProps& props) const {
     GrTexture* tex = this->getTexture();
     SkASSERT(tex);
@@ -49,6 +56,13 @@
 }
 
 bool SkImage_Gpu::getROPixels(SkBitmap* dst) const {
+    if (SkBitmapCache::Find(this->uniqueID(), dst)) {
+        SkASSERT(dst->getGenerationID() == this->uniqueID());
+        SkASSERT(dst->isImmutable());
+        SkASSERT(dst->getPixels());
+        return true;
+    }
+
     SkAlphaType at = this->isOpaque() ? kOpaque_SkAlphaType : kPremul_SkAlphaType;
     if (!dst->tryAllocPixels(SkImageInfo::MakeN32(this->width(), this->height(), at))) {
         return false;
@@ -57,6 +71,10 @@
                               dst->getPixels(), dst->rowBytes())) {
         return false;
     }
+
+    dst->pixelRef()->setImmutableWithID(this->uniqueID());
+    SkBitmapCache::Add(this->uniqueID(), *dst);
+    fAddedRasterVersionToCache.store(true);
     return true;
 }
 
diff --git a/src/image/SkImage_Gpu.h b/src/image/SkImage_Gpu.h
index 4c7ebd6..9481690 100644
--- a/src/image/SkImage_Gpu.h
+++ b/src/image/SkImage_Gpu.h
@@ -8,6 +8,7 @@
 #ifndef SkImage_Gpu_DEFINED
 #define SkImage_Gpu_DEFINED
 
+#include "SkAtomics.h"
 #include "GrTexture.h"
 #include "GrGpuResourcePriv.h"
 #include "SkBitmap.h"
@@ -17,14 +18,13 @@
 
 class SkImage_Gpu : public SkImage_Base {
 public:
-    
-
     /**
      *  An "image" can be a subset/window into a larger texture, so we explicit take the
      *  width and height.
      */
     SkImage_Gpu(int w, int h, uint32_t uniqueID, SkAlphaType, GrTexture*,
                 int sampleCountForNewSurfaces, SkSurface::Budgeted);
+    ~SkImage_Gpu() override;
 
     void applyBudgetDecision() const {
         GrTexture* tex = this->getTexture();
@@ -47,10 +47,12 @@
                       int srcX, int srcY) const override;
 
 private:
-    SkAutoTUnref<GrTexture> fTexture;
-    const int               fSampleCountForNewSurfaces;   // 0 if we don't know
-    const SkAlphaType       fAlphaType;
-    SkSurface::Budgeted     fBudgeted;
+    SkAutoTUnref<GrTexture>     fTexture;
+    const int                   fSampleCountForNewSurfaces;   // 0 if we don't know
+    const SkAlphaType           fAlphaType;
+    const SkSurface::Budgeted   fBudgeted;
+    mutable SkAtomic<bool>      fAddedRasterVersionToCache;
+
 
     typedef SkImage_Base INHERITED;
 };