some basic speed ups for SkRasterPipeline::append()

The new bench demos the speedup:
    SkRasterPipelineReuse_…
    …full 1x  …some 1.8x  …none 5.22x

Change-Id: I5e51fb4316ae04558710ce62560850584ccb4aea
Reviewed-on: https://skia-review.googlesource.com/17449
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

2 files changed, 11 insertions, 2 deletions

diff --git a/src/core/SkRasterPipeline.cpp b/src/core/SkRasterPipeline.cpp
index d307bdd118..ab404446e0 100644
--- a/src/core/SkRasterPipeline.cpp
+++ b/src/core/SkRasterPipeline.cpp
@@ -7,7 +7,13 @@
 
 #include "SkRasterPipeline.h"
 
-SkRasterPipeline::SkRasterPipeline() {}
+SkRasterPipeline::SkRasterPipeline(int size_hint) {
+    fStages.reserve(size_hint);
+}
+
+void SkRasterPipeline::rewind() {
+    fStages.clear();
+}
 
 void SkRasterPipeline::append(StockStage stage, void* ctx) {
     SkASSERT(stage != from_srgb);
diff --git a/src/core/SkRasterPipeline.h b/src/core/SkRasterPipeline.h
index a5f47c2203..3b2d617c69 100644
--- a/src/core/SkRasterPipeline.h
+++ b/src/core/SkRasterPipeline.h
@@ -107,7 +107,7 @@
 
 class SkRasterPipeline {
 public:
-    SkRasterPipeline();
+    SkRasterPipeline(int size_hint=0);
 
     enum StockStage {
     #define M(stage) stage,
@@ -136,6 +136,9 @@ public:
 
     bool empty() const { return fStages.empty(); }
 
+    // Cheaply reset all state so that empty() returns true.
+    void rewind();
+
 private:
     std::vector<Stage> fStages;
 };