CanOverflow -> ClampIfNeeded

Centralize the clamping in ClampIfNeeded, and switch it to clamp_1.

Change-Id: I056ac029d3ddc81e3f0f194f2b9c9924cad4caae
Reviewed-on: https://skia-review.googlesource.com/19401
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>

src/core/SkBlendMode.cpp

@@ -8,10 +8,6 @@
 #include "SkBlendModePriv.h"
 #include "SkRasterPipeline.h"
 
-bool SkBlendMode_CanOverflow(SkBlendMode mode) {
-    return mode == SkBlendMode::kPlus;
-}
-
 bool SkBlendMode_SupportsCoverageAsAlpha(SkBlendMode mode) {
     switch (mode) {
         case SkBlendMode::kDst:
@@ -65,7 +61,7 @@ bool SkBlendMode_AsCoeff(SkBlendMode mode, SkBlendModeCoeff* src, SkBlendModeCoe
     return true;
 }
 
-void SkBlendMode_AppendStages(SkBlendMode mode, SkRasterPipeline* p) {
+void SkBlendMode_AppendStagesNoClamp(SkBlendMode mode, SkRasterPipeline* p) {
     auto stage = SkRasterPipeline::srcover;
     switch (mode) {
         case SkBlendMode::kClear:    stage = SkRasterPipeline::clear; break;
@@ -103,6 +99,14 @@ void SkBlendMode_AppendStages(SkBlendMode mode, SkRasterPipeline* p) {
     p->append(stage);
 }
 
+void SkBlendMode_AppendClampIfNeeded(SkBlendMode mode, SkRasterPipeline* p) {
+    if (mode == SkBlendMode::kPlus) {
+        // Both clamp_a and clamp_1 would preserve premultiplication invariants here,
+        // so we pick clamp_1 for being a smidge faster.
+        p->append(SkRasterPipeline::clamp_1);
+    }
+}
+
 SkPM4f SkBlendMode_Apply(SkBlendMode mode, SkPM4f src, SkPM4f dst) {
     // special-case simple/common modes...
     switch (mode) {
@@ -123,9 +127,6 @@ SkPM4f SkBlendMode_Apply(SkBlendMode mode, SkPM4f src, SkPM4f dst) {
     p.append(SkRasterPipeline::move_src_dst);
     p.append(SkRasterPipeline::load_f32, &src_ctx);
     SkBlendMode_AppendStages(mode, &p);
-    if (SkBlendMode_CanOverflow(mode)) {
-        p.append(SkRasterPipeline::clamp_1);
-    }
     p.append(SkRasterPipeline::store_f32, &dst_ctx);
     p.run(0, 0, 1);
     return dst;

src/core/SkBlendModePriv.h

@@ -14,8 +14,15 @@
 class SkRasterPipeline;
 
 bool SkBlendMode_SupportsCoverageAsAlpha(SkBlendMode);
-bool SkBlendMode_CanOverflow(SkBlendMode);
-void SkBlendMode_AppendStages(SkBlendMode, SkRasterPipeline*);
+
+void SkBlendMode_AppendStagesNoClamp(SkBlendMode, SkRasterPipeline*);
+void SkBlendMode_AppendClampIfNeeded(SkBlendMode, SkRasterPipeline*);
+
+static inline void SkBlendMode_AppendStages(SkBlendMode mode, SkRasterPipeline* p) {
+    // Only plus clamps, so maybe append a clamping plus here instead of a second stage?
+    SkBlendMode_AppendStagesNoClamp(mode, p);
+    SkBlendMode_AppendClampIfNeeded(mode, p);
+}
 
 #if SK_SUPPORT_GPU
 #include "GrXferProcessor.h"

src/core/SkModeColorFilter.cpp

@@ -89,11 +89,7 @@ void SkModeColorFilter::onAppendStages(SkRasterPipeline* p,
 
     p->append(SkRasterPipeline::move_src_dst);
     p->append(SkRasterPipeline::constant_color, color);
-    auto mode = (SkBlendMode)fMode;
-    SkBlendMode_AppendStages(mode, p);
-    if (SkBlendMode_CanOverflow(mode)) {
-        p->append(SkRasterPipeline::clamp_a);
-    }
+    SkBlendMode_AppendStages(fMode, p);
 }
 
 sk_sp<SkColorFilter> SkModeColorFilter::onMakeColorSpace(SkColorSpaceXformer* xformer) const {

src/core/SkRasterPipelineBlitter.cpp

@@ -268,13 +268,11 @@ void SkRasterPipelineBlitter::append_store(SkRasterPipeline* p) const {
 }
 
 void SkRasterPipelineBlitter::append_blend(SkRasterPipeline* p) const {
-    SkBlendMode_AppendStages(fBlend, p);
+    SkBlendMode_AppendStagesNoClamp(fBlend, p);
 }
 
 void SkRasterPipelineBlitter::maybe_clamp(SkRasterPipeline* p) const {
-    if (SkBlendMode_CanOverflow(fBlend)) {
-        p->append(SkRasterPipeline::clamp_a);
-    }
+    SkBlendMode_AppendClampIfNeeded(fBlend, p);
 }
 
 void SkRasterPipelineBlitter::maybe_shade(int x, int y, int w) {

src/shaders/SkComposeShader.cpp

@@ -110,12 +110,7 @@ bool SkComposeShader::onAppendStages(SkRasterPipeline* pipeline, SkColorSpace* d
     pipeline->append(SkRasterPipeline::load_rgba, storage->fRGBA);
 
     if (!this->isJustLerp()) {
-        // Idea: should time this, and see if it helps to have custom versions of the overflow modes
-        //       that do their own clamping, avoiding the overhead of an extra stage.
         SkBlendMode_AppendStages(fMode, pipeline);
-        if (SkBlendMode_CanOverflow(fMode)) {
-            pipeline->append(SkRasterPipeline::clamp_a);
-        }
     }
     if (!this->isJustMode()) {
         pipeline->append(SkRasterPipeline::lerp_1_float, &fLerpT);