Potentially optimize some GrGLEffects for known input color values

src/core/SkXfermode.cpp

Index: src/core/SkXfermode.cpp
diff --git a/src/core/SkXfermode.cpp b/src/core/SkXfermode.cpp
index 993c754711ac472fd3bc49b6fd01ae578d520fd8..45e83c80a2981a0573d23a756236325af1d1661f 100644
--- a/src/core/SkXfermode.cpp
+++ b/src/core/SkXfermode.cpp
@@ -885,6 +885,9 @@ void SkProcXfermode::toString(SkString* str) const {
 #include "GrTBackendEffectFactory.h"
 #include "gl/GrGLEffect.h"
 
+template GrGLSLExpr3 GrGLSL::min<GrGLSLExpr3>(const GrGLSLExpr3&, const GrGLSLExpr3&);
+template GrGLSLExpr3 GrGLSL::max<GrGLSLExpr3>(const GrGLSLExpr3&, const GrGLSLExpr3&);
+
 /**
  * GrEffect that implements the all the separable xfer modes that cannot be expressed as Coeffs.
  */
@@ -926,7 +929,7 @@ public:
                               const GrDrawEffect& drawEffect,
                               EffectKey key,
                               const char* outputColor,
-                              const char* inputColor,
+                              const GrGLSLExpr4& inputColorExpr,
                               const TransformedCoordsArray& coords,
                               const TextureSamplerArray& samplers) SK_OVERRIDE {
             SkXfermode::Mode mode = drawEffect.castEffect<XferEffect>().mode();
@@ -942,16 +945,16 @@ public:
             }
             SkASSERT(NULL != dstColor);
 
-            // We don't try to optimize for this case at all
-            if (NULL == inputColor) {
-                builder->fsCodeAppendf("\t\tconst vec4 ones = vec4(1);\n");
-                inputColor = "ones";
-            }
             builder->fsCodeAppendf("\t\t// SkXfermode::Mode: %s\n", SkXfermode::ModeName(mode));
 
+            GrGLSLExpr1 one(1);
+            GrGLSLExpr4 dstColorExpr(dstColor);
             // These all perform src-over on the alpha channel.
-            builder->fsCodeAppendf("\t\t%s.a = %s.a + (1.0 - %s.a) * %s.a;\n",
-                                    outputColor, inputColor, inputColor, dstColor);
+            builder->fsCodeAppendf("\t\t%s.a = %s;\n",
+                                   outputColor,
+                                   (inputColorExpr.a() + (one - inputColorExpr.a()) * dstColorExpr.a()).c_str());
+
+            const char* inputColor = inputColorExpr.c_str(); // Shorthand for below.
 
             switch (mode) {
                 case SkXfermode::kOverlay_Mode:
@@ -959,19 +962,15 @@ public:
                     HardLight(builder, outputColor, dstColor, inputColor);
                     break;
                 case SkXfermode::kDarken_Mode:
-                    builder->fsCodeAppendf("\t\t%s.rgb = min((1.0 - %s.a) * %s.rgb + %s.rgb, "
-                                                            "(1.0 - %s.a) * %s.rgb + %s.rgb);\n",
-                                            outputColor,
-                                            inputColor, dstColor, inputColor,
-                                            dstColor, inputColor, dstColor);
-                    break;
-                case SkXfermode::kLighten_Mode:
-                    builder->fsCodeAppendf("\t\t%s.rgb = max((1.0 - %s.a) * %s.rgb + %s.rgb, "
-                                                            "(1.0 - %s.a) * %s.rgb + %s.rgb);\n",
-                                            outputColor,
-                                            inputColor, dstColor, inputColor,
-                                            dstColor, inputColor, dstColor);
+                case SkXfermode::kLighten_Mode: {
+                    GrGLSLExpr3 (*minOrMax)(const GrGLSLExpr3&, const GrGLSLExpr3&);
+                    minOrMax = mode == SkXfermode::kDarken_Mode ? &GrGLSL::min<GrGLSLExpr3> : &GrGLSL::max<GrGLSLExpr3>;
+
+                    builder->fsCodeAppendf("\t\t%s.rgb = %s;\n", outputColor,
+                                           minOrMax((one - inputColorExpr.a()) * dstColorExpr.rgb() + inputColorExpr.rgb(),
+                                                (one - dstColorExpr.a()) *  inputColorExpr.rgb() + dstColorExpr.rgb()).c_str());
                     break;
+                }
                 case SkXfermode::kColorDodge_Mode:
                     ColorDodgeComponent(builder, outputColor, inputColor, dstColor, 'r');
                     ColorDodgeComponent(builder, outputColor, inputColor, dstColor, 'g');