Avoid re-rendering stencil clip for every draw with reducable clip stack

src/gpu/GrReducedClip.cpp

Index: src/gpu/GrReducedClip.cpp
diff --git a/src/gpu/GrReducedClip.cpp b/src/gpu/GrReducedClip.cpp
index 2b6583e3e328f2a5454620a3c2ae68b3a992c7de..9a7fc97529566b3a6324bcd35c0df07775875e03 100644
--- a/src/gpu/GrReducedClip.cpp
+++ b/src/gpu/GrReducedClip.cpp
@@ -70,7 +70,9 @@ void ReduceClipStack(const SkClipStack& stack,
                 SkRect scalarTighterBounds = SkRect::MakeFromIRect(*tighterBounds);
                 if (scalarTighterBounds == isectRect) {
                     // the round-out didn't add any area outside the clip rect.
-                    *requiresAA = false;
+                    if (NULL != requiresAA) {
+                        *requiresAA = false;
+                    }
                     *initialState = kAllIn_InitialState;
                     return;
                 }
@@ -78,7 +80,12 @@ void ReduceClipStack(const SkClipStack& stack,
                 // iior should only be true if aa/non-aa status matches among all elements.
                 SkClipStack::Iter iter(stack, SkClipStack::Iter::kTop_IterStart);
                 bool doAA = iter.prev()->isAA();
-                SkNEW_INSERT_AT_LLIST_HEAD(result, Element, (isectRect, SkRegion::kReplace_Op, doAA));
+                // The clip established by the element list might be cached based on the last
+                // generation id. Without walking the stack, we do not knoow what was the generation
+                // id that lead to this state. Make a conservative guess.
+                SkNEW_INSERT_AT_LLIST_HEAD(result, Element, (isectRect, SkRegion::kReplace_Op, doAA,

[bsalomon, 2013/10/30 14:25:59]

Rather than assigning a genID to the manufactured element maybe this function
could just take an int32_t* genID param that gets updated?

Then we wouldn't need a public Element constructor that takes a genID. That
seems a like an avenue for someone to introduce future bugs.

[Kimmo Kinnunen, 2013/11/01 12:12:20]

Done.

+                                                             stack.getTopmostGenID()));
+
                 if (NULL != requiresAA) {
                     *requiresAA = doAA;
                 }
@@ -124,6 +131,10 @@ void ReduceClipStack(const SkClipStack& stack,
     // Now that we have determined the bounds to use and filtered out the trivial cases, call the
     // helper that actually walks the stack.
     reduced_stack_walker(stack, scalarBounds, result, initialState, requiresAA);
+
+    // The list that was computed in this function may be cached based on the gen id of the last
+    // element.
+    SkASSERT(0 == result->count() || SkClipStack::kInvalidGenID  != result->tail()->getGenID());
 }
 
 void reduced_stack_walker(const SkClipStack& stack,
@@ -318,7 +329,7 @@ void reduced_stack_walker(const SkClipStack& stack,
                          SkRegion::kReverseDifference_Op == element->getOp());
                 SkNEW_INSERT_AT_LLIST_HEAD(result,
                                            Element,
-                                           (queryBounds, SkRegion::kReverseDifference_Op, false));
+                                           (queryBounds, SkRegion::kReverseDifference_Op, false, element->getGenID()));
             } else {
                 Element* newElement = result->addToHead(*element);
                 if (newElement->isAA()) {