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

src/gpu/GrReducedClip.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 "GrReducedClip.h"
 
 typedef SkClipStack::Element Element;
 ////////////////////////////////////////////////////////////////////////////////
 
 namespace GrReducedClip {
 
 // helper function
 void reduced_stack_walker(const SkClipStack& stack,
                           const SkRect& queryBounds,
                           ElementList* result,
+                          int32_t* resultGenID,
                           InitialState* initialState,
                           bool* requiresAA);
 
 /*
 There are plenty of optimizations that could be added here. Maybe flips could be folded into
 earlier operations. Or would inserting flips and reversing earlier ops ever be a win? Perhaps
 for the case where the bounds are kInsideOut_BoundsType. We could restrict earlier operations
 based on later intersect operations, and perhaps remove intersect-rects. We could optionally
 take a rect in case the caller knows a bound on what is to be drawn through this clip.
 */
 void ReduceClipStack(const SkClipStack& stack,
                      const SkIRect& queryBounds,
                      ElementList* result,
+                     int32_t* resultGenID,
                      InitialState* initialState,
                      SkIRect* tighterBounds,
                      bool* requiresAA) {
     result->reset();
 
+    // The clip established by the element list might be cached based on the last
+    // generation id. When we make early returns, we do not know what was the generation
+    // id that lead to the state. Make a conservative guess.
+    *resultGenID = stack.getTopmostGenID();
+
     if (stack.isWideOpen()) {
         *initialState = kAllIn_InitialState;
         return;
     }
 
 
     // We initially look at whether the bounds alone is sufficient. We also use the stack bounds to
     // attempt to compute the tighterBounds.
 
     SkClipStack::BoundsType stackBoundsType;
@@ skipping 15 matching lines @@
             }
             if (NULL != requiresAA) {
                *requiresAA = false;
             }
         } else if (isectRect.intersect(stackBounds, scalarQueryBounds)) {
             if (NULL != tighterBounds) {
                 isectRect.roundOut(tighterBounds);
                 SkRect scalarTighterBounds = SkRect::Make(*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;
                 }
                 *initialState = kAllOut_InitialState;
                 // 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));
                 if (NULL != requiresAA) {
                     *requiresAA = doAA;
@@ skipping 32 matching lines @@
             if (NULL != tighterBounds) {
                 *tighterBounds = queryBounds;
             }
         }
     }
 
     SkRect scalarBounds = SkRect::Make(*bounds);
 
     // 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);
+    reduced_stack_walker(stack, scalarBounds, result, resultGenID, initialState, requiresAA);
+
+    // The list that was computed in this function may be cached based on the gen id of the last
+    // element.
+    SkASSERT(SkClipStack::kInvalidGenID != *resultGenID);
 }
 
 void reduced_stack_walker(const SkClipStack& stack,
                           const SkRect& queryBounds,
                           ElementList* result,
+                          int32_t* resultGenID,
                           InitialState* initialState,
                           bool* requiresAA) {
 
     // walk backwards until we get to:
     //  a) the beginning
     //  b) an operation that is known to make the bounds all inside/outside
     //  c) a replace operation
 
     static const InitialState kUnknown_InitialState = static_cast<InitialState>(-1);
     *initialState = kUnknown_InitialState;
@@ skipping 163 matching lines @@
                 if (!skippable) {
                     *initialState = kAllOut_InitialState;
                     embiggens = emsmallens = true;
                 }
                 break;
             default:
                 SkDEBUGFAIL("Unexpected op.");
                 break;
         }
         if (!skippable) {
+            if (0 == result->count()) {
+                // This will be the last element. Record the stricter genID.
+                *resultGenID = element->getGenID();
+            }
+
             // if it is a flip, change it to a bounds-filling rect
             if (isFlip) {
                 SkASSERT(SkRegion::kXOR_Op == element->getOp() ||
                          SkRegion::kReverseDifference_Op == element->getOp());
                 SkNEW_INSERT_AT_LLIST_HEAD(result,
                                            Element,
                                            (queryBounds, SkRegion::kReverseDifference_Op, false));
             } else {
                 Element* newElement = result->addToHead(*element);
                 if (newElement->isAA()) {
@@ skipping 85 matching lines @@
                     --numAAElements;
                 }
                 result->popHead();
                 element = result->headIter().get();
             }
         }
     }
     if (NULL != requiresAA) {
         *requiresAA = numAAElements > 0;
     }
+
+    if (0 == result->count()) {
+        if (*initialState == kAllIn_InitialState) {
+            *resultGenID = SkClipStack::kWideOpenGenID;
+        } else {
+            *resultGenID = SkClipStack::kEmptyGenID;
+        }
+    }
 }
 } // namespace GrReducedClip