Remove the AA requirement for selecting GrEffect-based clipping.

src/gpu/GrClipMaskManager.cpp

Index: src/gpu/GrClipMaskManager.cpp
diff --git a/src/gpu/GrClipMaskManager.cpp b/src/gpu/GrClipMaskManager.cpp
index a878894c59949410cf2c0522e766657f23c06336..d3c0cf3fc140ec3cc1c6cd4a6816bb41f4e4202a 100644
--- a/src/gpu/GrClipMaskManager.cpp
+++ b/src/gpu/GrClipMaskManager.cpp
@@ -110,7 +110,10 @@ bool GrClipMaskManager::useSWOnlyPath(const ElementList& elements) {
 bool GrClipMaskManager::installClipEffects(const ElementList& elements,
                                            GrDrawState::AutoRestoreEffects* are,
                                            const SkVector& clipToRTOffset,
-                                           const SkRect* drawBounds) {
+                                           const SkRect* drawBounds,
+                                           SkIRect* scissorRect) {
+
+    SkASSERT(NULL != scissorRect);
 
     GrDrawState* drawState = fGpu->drawState();
     SkRect boundsInClipSpace;
@@ -121,7 +124,11 @@ bool GrClipMaskManager::installClipEffects(const ElementList& elements,
 
     are->set(drawState);
     GrRenderTarget* rt = drawState->getRenderTarget();
-    ElementList::Iter iter(elements);
+    // We iterate from the top of the stack to the bottom. We do this because we select the first
+    // BW rectangle as the scissor. Clients performing hierarchical rendering tend to use smaller
+    // clips towards the top of the clip stack. Smaller scissor rects can help tiled architectures
+    // skip processing tiles for draws.
+    ElementList::Iter iter(elements, ElementList::Iter::kTail_IterStart);
 
     bool setARE = false;
     bool failed = false;
@@ -157,7 +164,7 @@ bool GrClipMaskManager::installClipEffects(const ElementList& elements,
             GrEffectEdgeType edgeType;
             if (GR_AA_CLIP && iter.get()->isAA()) {
                 if (rt->isMultisampled()) {
-                    // Coverage based AA clips don't place nicely with MSAA.
+                    // Coverage based AA clips don't play nicely with MSAA.
                     failed = true;
                     break;
                 }
@@ -165,6 +172,8 @@ bool GrClipMaskManager::installClipEffects(const ElementList& elements,
             } else {
                 edgeType = invert ? kInverseFillBW_GrEffectEdgeType : kFillBW_GrEffectEdgeType;
             }
+            // We don't want to exit if we convert a BW rect clip to a scissor.
+            bool failIfNoEffect = true;
             SkAutoTUnref<GrEffectRef> effect;
             switch (iter.get()->getType()) {
                 case SkClipStack::Element::kPath_Type:
@@ -180,7 +189,14 @@ bool GrClipMaskManager::installClipEffects(const ElementList& elements,
                 case SkClipStack::Element::kRect_Type: {
                     SkRect rect = iter.get()->getRect();
                     rect.offset(clipToRTOffset.fX, clipToRTOffset.fY);
-                    effect.reset(GrConvexPolyEffect::Create(edgeType, rect));
+                    if (kFillBW_GrEffectEdgeType == edgeType && scissorRect->isEmpty()) {
+                        // This is OK because we only allow clip operations that shrink the clip
+                        // to be implemented as effects.
+                        rect.roundOut(scissorRect);
+                        failIfNoEffect = false;
+                    } else {
+                        effect.reset(GrConvexPolyEffect::Create(edgeType, rect));
+                    }
                     break;
                 }
                 default:
@@ -192,12 +208,12 @@ bool GrClipMaskManager::installClipEffects(const ElementList& elements,
                     setARE = true;
                 }
                 fGpu->drawState()->addCoverageEffect(effect);
-            } else {
+            } else if (failIfNoEffect) {
                 failed = true;
                 break;
             }
         }
-        iter.next();
+        iter.prev();
     }
 
     if (failed) {
@@ -207,6 +223,12 @@ bool GrClipMaskManager::installClipEffects(const ElementList& elements,
     return !failed;
 }
 
+static inline bool rect_contains_irect(const SkIRect ir, const SkRect& r) {
+    SkASSERT(!ir.isEmpty());
+    return ir.fLeft <= r.fLeft && ir.fTop <= r.fTop &&
+           ir.fRight >= r.fRight && ir.fBottom >= r.fBottom;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // sort out what kind of clip mask needs to be created: alpha, stencil,
 // scissor, or entirely software
@@ -263,17 +285,32 @@ bool GrClipMaskManager::setupClipping(const GrClipData* clipDataIn,
     // configuration's relative costs of switching RTs to generate a mask vs
     // longer shaders.
     if (elements.count() <= 4) {
+        SkIRect scissorRect;
+        scissorRect.setEmpty();
         SkVector clipToRTOffset = { SkIntToScalar(-clipDataIn->fOrigin.fX),
                                     SkIntToScalar(-clipDataIn->fOrigin.fY) };
         if (elements.isEmpty() ||
-            (requiresAA && this->installClipEffects(elements, are, clipToRTOffset, devBounds))) {
-            SkIRect scissorSpaceIBounds(clipSpaceIBounds);
-            scissorSpaceIBounds.offset(-clipDataIn->fOrigin);
-            if (NULL == devBounds ||
-                !SkRect::Make(scissorSpaceIBounds).contains(*devBounds)) {
-                fGpu->enableScissor(scissorSpaceIBounds);
+            this->installClipEffects(elements, are, clipToRTOffset, devBounds, &scissorRect)) {
+            if (scissorRect.isEmpty()) {
+                // We may still want to use a scissor, especially on tiled architectures.
+                scissorRect = clipSpaceIBounds;
+                scissorRect.offset(-clipDataIn->fOrigin);
+                if (NULL == devBounds ||
+                    !rect_contains_irect(scissorRect, *devBounds)) {
+                    fGpu->enableScissor(scissorRect);
+                } else {
+                    // When the vertices that will be rendered fit fully inside the clip's bounds
+                    // then providing the scissor rect will not help the driver eliminate tiles
+                    // from consideration for the draw, but changing the scissor will cause
+                    // state changes between draws.
+                    fGpu->disableScissor();
+                }
             } else {
-                fGpu->disableScissor();
+                scissorRect.fLeft = SkTMax(0, scissorRect.fLeft);
+                scissorRect.fTop = SkTMax(0, scissorRect.fTop);
+                scissorRect.fRight = SkTMin(rt->width(), scissorRect.fRight);
+                scissorRect.fBottom = SkTMin(rt->height(), scissorRect.fBottom);
+                fGpu->enableScissor(scissorRect);
             }
             this->setGpuStencil();
             return true;