Add SaveLayer-Draw-Restore optimization.

src/record/SkRecordOpts.cpp

Index: src/record/SkRecordOpts.cpp
diff --git a/src/record/SkRecordOpts.cpp b/src/record/SkRecordOpts.cpp
index aaa611cf392cf6126b5548f9c5f4e1bcb6a46e59..5b648f4215e5bcfae6e2878981b0e8b28b7ed948 100644
--- a/src/record/SkRecordOpts.cpp
+++ b/src/record/SkRecordOpts.cpp
@@ -16,6 +16,7 @@ using namespace SkRecords;
 void SkRecordOptimize(SkRecord* record) {
     // TODO(mtklein): fuse independent optimizations to reduce number of passes?
     SkRecordNoopSaveRestores(record);
+    SkRecordNoopSaveLayerDrawRestores(record);
     SkRecordAnnotateCullingPairs(record);
     SkRecordReduceDrawPosTextStrength(record);  // Helpful to run this before BoundDrawPosTextH.
     SkRecordBoundDrawPosTextH(record);
@@ -69,6 +70,73 @@ void SkRecordNoopSaveRestores(SkRecord* record) {
     while (apply(&pass, record));  // Run until it stops changing things.
 }
 
+// For some SaveLayer-[drawing command]-Restore patterns, merge the SaveLayer's alpha into the
+// draw, and no-op the SaveLayer and Restore.
+struct SaveLayerDrawRestoreNooper {
+    typedef Pattern3<Is<SaveLayer>, IsDraw, Is<Restore> > Pattern;
+
+    bool onMatch(SkRecord* record, Pattern* pattern, unsigned begin, unsigned end) {
+        SaveLayer* saveLayer = pattern->first<SaveLayer>();
+        if (saveLayer->bounds != NULL) {
+            // SaveLayer with bounds is too tricky for us.
+            return false;
+        }
+
+        SkPaint* layerPaint = saveLayer->paint;
+        if (NULL == layerPaint) {
+            // There wasn't really any point to this SaveLayer at all.
+            return KillSaveLayerAndRestore(record, begin);
+        }
+
+        SkPaint* drawPaint = pattern->second<SkPaint>();
+        if (drawPaint == NULL) {
+            // We can just give the draw the SaveLayer's paint.
+            // TODO(mtklein): figure out how to do this clearly
+            return false;
+        }
+
+        const uint32_t layerColor = layerPaint->getColor();
+        const uint32_t  drawColor =  drawPaint->getColor();
+        if (!IsOnlyAlpha(layerColor) || !IsOpaque(drawColor) || HasAnyEffect(*layerPaint)) {
+            // Too fancy for us.  Actually, as long as layerColor is just an alpha
+            // we can blend it into drawColor's alpha; drawColor doesn't strictly have to be opaque.
+            return false;
+        }
+
+        drawPaint->setColor(SkColorSetA(drawColor, SkColorGetA(layerColor)));
+        return KillSaveLayerAndRestore(record, begin);
+    }
+
+    static bool KillSaveLayerAndRestore(SkRecord* record, unsigned saveLayerIndex) {
+        record->replace<NoOp>(saveLayerIndex);    // SaveLayer
+        record->replace<NoOp>(saveLayerIndex+2);  // Restore
+        return true;
+    }
+
+    static bool HasAnyEffect(const SkPaint& paint) {
+        return paint.getPathEffect()  ||
+               paint.getShader()      ||
+               paint.getXfermode()    ||
+               paint.getMaskFilter()  ||
+               paint.getColorFilter() ||
+               paint.getRasterizer()  ||
+               paint.getLooper()      ||
+               paint.getImageFilter();
+    }
+
+    static bool IsOpaque(SkColor color) {
+        return SkColorGetA(color) == SK_AlphaOPAQUE;
+    }
+    static bool IsOnlyAlpha(SkColor color) {
+        return SK_ColorTRANSPARENT == SkColorSetA(color, SK_AlphaTRANSPARENT);
+    }
+};
+void SkRecordNoopSaveLayerDrawRestores(SkRecord* record) {
+    SaveLayerDrawRestoreNooper pass;
+    apply(&pass, record);
+}
+
+
 // Replaces DrawPosText with DrawPosTextH when all Y coordinates are equal.
 struct StrengthReducer {
     typedef Pattern1<Is<DrawPosText> > Pattern;