Implement difference clip rects with window rectangles

src/gpu/GrClipStackClip.cpp

Index: src/gpu/GrClipStackClip.cpp
diff --git a/src/gpu/GrClipStackClip.cpp b/src/gpu/GrClipStackClip.cpp
index a3a0de509c0db3e1567d195662a174e30f7d811a..9428d254327b386aedb2d20827de93a3b6618ee1 100644
--- a/src/gpu/GrClipStackClip.cpp
+++ b/src/gpu/GrClipStackClip.cpp
@@ -10,6 +10,7 @@
 #include "GrDrawingManager.h"
 #include "GrDrawContextPriv.h"
 #include "GrGpuResourcePriv.h"
+#include "GrRenderTargetPriv.h"
 #include "GrStencilAttachment.h"
 #include "GrSWMaskHelper.h"
 #include "effects/GrConvexPolyEffect.h"
@@ -280,6 +281,22 @@ bool GrClipStackClip::apply(GrContext* context, GrDrawContext* drawContext, bool
 
     SkASSERT(reducedClip.hasIBounds());
 
+    // Attempt to implement difference clip rects with window rectangles. This will eventually
+    // become more comprehensive.
+    if (drawContext->accessRenderTarget()->renderTargetPriv().supportsWindowRectangles() &&
+        1 == reducedClip.elements().count() && !reducedClip.requiresAA() &&
+        InitialState::kAllIn == reducedClip.initialState()) {
+        const Element* element = reducedClip.elements().head();
+        SkRegion::Op op = element->getOp();
+        if (Element::kRect_Type == element->getType() &&
+            (SkRegion::kDifference_Op == op || SkRegion::kXOR_Op == op)) {
+            SkIRect window;
+            element->getRect().round(&window);

[csmartdalton, 2016/08/16 18:38:47]

Needs to be offset to screen space from clip space.

[csmartdalton, 2016/08/16 23:38:34]

Done.

+            out->addWindowRectangle(window, *context->caps());
+            return true;
+        }
+    }
+
     // An element count of 4 was chosen because of the common pattern in Blink of:
     //   isect RR
     //   diff  RR