Adding optimization to avoid image copy in SkSurface copy on write when content is discardable

src/image/SkSurface_Gpu.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Base.h"
 #include "SkImagePriv.h"
 #include "SkCanvas.h"
 #include "SkGpuDevice.h"
 
 class SkSurface_Gpu : public SkSurface_Base {
 public:
     SK_DECLARE_INST_COUNT(SkSurface_Gpu)
 
     SkSurface_Gpu(GrContext*, const SkImage::Info&, int sampleCount);
     SkSurface_Gpu(GrContext*, GrRenderTarget*);
     virtual ~SkSurface_Gpu();
 
     virtual SkCanvas* onNewCanvas() SK_OVERRIDE;
     virtual SkSurface* onNewSurface(const SkImage::Info&) SK_OVERRIDE;
     virtual SkImage* onNewImageSnapshot() SK_OVERRIDE;
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y,
                         const SkPaint*) SK_OVERRIDE;
-    virtual void onCopyOnWrite() SK_OVERRIDE;
+    virtual void onCopyOnWrite(ContentChangeMode) SK_OVERRIDE;
 
 private:
     SkGpuDevice* fDevice;
 
     typedef SkSurface_Base INHERITED;
 };
 
 SK_DEFINE_INST_COUNT(SkSurface_Gpu)
 
 ///////////////////////////////////////////////////////////////////////////////
@@ skipping 42 matching lines @@
 }
 
 void SkSurface_Gpu::onDraw(SkCanvas* canvas, SkScalar x, SkScalar y,
                               const SkPaint* paint) {
     canvas->drawBitmap(fDevice->accessBitmap(false), x, y, paint);
 }
 
 // Create a new SkGpuDevice and, if necessary, copy the contents of the old
 // device into it. Note that this flushes the SkGpuDevice but
 // doesn't force an OpenGL flush.
-void SkSurface_Gpu::onCopyOnWrite() {
+void SkSurface_Gpu::onCopyOnWrite(ContentChangeMode mode) {
     GrRenderTarget* rt = (GrRenderTarget*) fDevice->accessRenderTarget();
-
     // are we sharing our render target with the image?
     SkASSERT(NULL != this->getCachedImage());
     if (rt->asTexture() == SkTextureImageGetTexture(this->getCachedImage())) {
         SkGpuDevice* newDevice = static_cast<SkGpuDevice*>(
             fDevice->createCompatibleDevice(fDevice->config(), fDevice->width(),
             fDevice->height(), fDevice->isOpaque()));
         SkAutoTUnref<SkGpuDevice> aurd(newDevice);
-        fDevice->context()->copyTexture(rt->asTexture(),
-            (GrRenderTarget*)newDevice->accessRenderTarget());
+        if (kRetain_ContentChangeMode == mode) {
+            fDevice->context()->copyTexture(rt->asTexture(),
+                reinterpret_cast<GrRenderTarget*>(newDevice->accessRenderTarget()));
+        }
         SkASSERT(NULL != this->getCachedCanvas());
         SkASSERT(this->getCachedCanvas()->getDevice() == fDevice);
         this->getCachedCanvas()->setDevice(newDevice);
         SkRefCnt_SafeAssign(fDevice, newDevice);
     }
 }
 
 ///////////////////////////////////////////////////////////////////////////////
 
 SkSurface* SkSurface::NewRenderTargetDirect(GrContext* ctx,
@@ skipping 20 matching lines @@
     desc.fConfig = SkBitmapConfig2GrPixelConfig(config);
     desc.fSampleCnt = sampleCount;
 
     SkAutoTUnref<GrTexture> tex(ctx->createUncachedTexture(desc, NULL, 0));
     if (NULL == tex) {
         return NULL;
     }
 
     return SkNEW_ARGS(SkSurface_Gpu, (ctx, tex->asRenderTarget()));
 }