Add SkRasterPipeline support to SkModeColorFilter.

The shader leaves its color in r,g,b,a, so to implement this color filter, we move {r,g,b,a} into {dr,dg,db,da}, then load the filter's color in {r,g,b,a}, then apply the xfermode as usual.

I've left a note about how we could sometimes cut a stage for some xfermodes.  Similarly we really only need to move_src_dst instead of swap_src_dst, but it seemed handy and less error prone to do a full two way swap.  As usual, we can always circle back and fine-tune these things if we want.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3243
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Change-Id: I928c0fb25236eb75cf238134c6bebb53af5ddf07
Reviewed-on: https://skia-review.googlesource.com/3243
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>

src/core/SkModeColorFilter.cpp

@@ -9,6 +9,7 @@
 #include "SkColorFilter.h"
 #include "SkColorPriv.h"
 #include "SkModeColorFilter.h"
+#include "SkRasterPipeline.h"
 #include "SkReadBuffer.h"
 #include "SkWriteBuffer.h"
 #include "SkUtils.h"
@@ -59,11 +60,10 @@ void SkModeColorFilter::filterSpan(const SkPMColor shader[], int count, SkPMColo
 }
 
 void SkModeColorFilter::filterSpan4f(const SkPM4f shader[], int count, SkPM4f result[]) const {
-    SkPM4f            color = SkPM4f::FromPMColor(fPMColor);
     SkXfermodeProc4f  proc = SkXfermode::GetProc4f(fMode);
 
     for (int i = 0; i < count; i++) {
-        result[i] = proc(color, shader[i]);
+        result[i] = proc(fPM4f, shader[i]);
     }
 }
 
@@ -75,6 +75,7 @@ void SkModeColorFilter::flatten(SkWriteBuffer& buffer) const {
 void SkModeColorFilter::updateCache() {
     fPMColor = SkPreMultiplyColor(fColor);
     fProc = SkXfermode::GetProc(fMode);
+    fPM4f = SkPM4f::FromPMColor(fPMColor);
 }
 
 sk_sp<SkFlattenable> SkModeColorFilter::CreateProc(SkReadBuffer& buffer) {
@@ -83,6 +84,20 @@ sk_sp<SkFlattenable> SkModeColorFilter::CreateProc(SkReadBuffer& buffer) {
     return SkColorFilter::MakeModeFilter(color, mode);
 }
 
+bool SkModeColorFilter::onAppendStages(SkRasterPipeline* p) const {
+    // TODO: For some modes we can cut a stage by loading the fPM4f into dr,dg,db,da
+    // and applying the opposite xfermode, e.g. dst-in instead of src-in.
+    p->append(SkRasterPipeline::swap_src_dst);
+    p->append(SkRasterPipeline::constant_color, &fPM4f);
+
+    // TODO: This is ugly.  I think we want static SkXfermode::AppendStages(Mode).
+    if (auto xfermode = SkXfermode::Make(fMode)) {
+        return xfermode->appendStages(p);
+    }
+    p->append(SkRasterPipeline::srcover);
+    return true;
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 #if SK_SUPPORT_GPU
 #include "GrBlend.h"

src/core/SkModeColorFilter.h

@@ -6,6 +6,7 @@
  */
 
 #include "SkColorFilter.h"
+#include "SkPM4f.h"
 #include "SkXfermode.h"
 
 #ifndef SkModeColorFilter_DEFINED
@@ -49,12 +50,15 @@ protected:
 
     void flatten(SkWriteBuffer&) const override;
 
+    bool onAppendStages(SkRasterPipeline*) const override;
+
 private:
     SkColor             fColor;
     SkXfermode::Mode    fMode;
     // cache
     SkPMColor           fPMColor;
     SkXfermodeProc      fProc;
+    SkPM4f              fPM4f;
 
     void updateCache();
 

src/core/SkOpts.cpp

@@ -94,6 +94,7 @@ namespace SkOpts {
 
     SkOpts::VoidFn body[] = {
         (SkOpts::VoidFn)SK_OPTS_NS::just_return,
+        (SkOpts::VoidFn)SK_OPTS_NS::swap_src_dst,
 
         (SkOpts::VoidFn)SK_OPTS_NS::store_565,
         (SkOpts::VoidFn)SK_OPTS_NS::store_srgb,
@@ -144,6 +145,7 @@ namespace SkOpts {
 
     SkOpts::VoidFn tail[] = {
         (SkOpts::VoidFn)SK_OPTS_NS::just_return,
+        (SkOpts::VoidFn)SK_OPTS_NS::swap_src_dst,
 
         (SkOpts::VoidFn)SK_OPTS_NS::store_565_tail,
         (SkOpts::VoidFn)SK_OPTS_NS::store_srgb_tail,

src/core/SkRasterPipeline.h

@@ -91,6 +91,7 @@ public:
 
     enum StockStage {
         just_return,
+        swap_src_dst,
 
         store_565,
         store_srgb,

src/opts/SkRasterPipeline_opts.h

@@ -152,6 +152,13 @@ namespace SK_OPTS_NS {
 
     STAGE(just_return, false) { }
 
+    STAGE(swap_src_dst, true) {
+        SkTSwap(r,dr);
+        SkTSwap(g,dg);
+        SkTSwap(b,db);
+        SkTSwap(a,da);
+    }
+
     // The default shader produces a constant color (from the SkPaint).
     STAGE(constant_color, true) {
         auto color = (const SkPM4f*)ctx;