Implement difference clip rects with window rectangles

src/gpu/GrClipStackClip.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrClipStackClip.h"
 
+#include "GrAppliedClip.h"
 #include "GrDrawingManager.h"
 #include "GrDrawContextPriv.h"
 #include "GrGpuResourcePriv.h"
+#include "GrRenderTargetPriv.h"
 #include "GrStencilAttachment.h"
 #include "GrSWMaskHelper.h"
 #include "effects/GrConvexPolyEffect.h"
 #include "effects/GrRRectEffect.h"
 #include "effects/GrTextureDomain.h"
 
 typedef SkClipStack::Element Element;
 typedef GrReducedClip::InitialState InitialState;
 typedef GrReducedClip::ElementList ElementList;
 
@@ skipping 250 matching lines @@
         scissorSpaceIBounds.offset(-fOrigin);
         out->addScissor(scissorSpaceIBounds);
     }
 
     if (reducedClip.elements().isEmpty()) {
         return InitialState::kAllIn == reducedClip.initialState();
     }
 
     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);
+            window.offset(-fOrigin);
+            out->addWindowRectangle(window);
+            return true;
+        }
+    }
+
     // An element count of 4 was chosen because of the common pattern in Blink of:
     //   isect RR
     //   diff  RR
     //   isect convex_poly
     //   isect convex_poly
     // when drawing rounded div borders. This could probably be tuned based on a
     // configuration's relative costs of switching RTs to generate a mask vs
     // longer shaders.
     if (reducedClip.elements().count() <= kMaxAnalyticElements) {
         // When there are multiple samples we want to do per-sample clipping, not compute a
@@ skipping 470 matching lines @@
     sk_sp<GrTexture> result(texProvider->createApproxTexture(desc));
     if (!result) {
         return nullptr;
     }
     result->resourcePriv().setUniqueKey(key);
 
     helper.toTexture(result.get());
 
     return result;
 }