Potentially optimize some GrGLEffects for known input color values

src/effects/SkArithmeticMode.cpp

Index: src/effects/SkArithmeticMode.cpp
diff --git a/src/effects/SkArithmeticMode.cpp b/src/effects/SkArithmeticMode.cpp
index fff03c291fb27550a44ead6dae5164deabbd3bcc..e47a7ac278467d3b91a862302987aaa7b872d569 100644
--- a/src/effects/SkArithmeticMode.cpp
+++ b/src/effects/SkArithmeticMode.cpp
@@ -245,7 +245,7 @@ public:
                           const GrDrawEffect&,
                           EffectKey,
                           const char* outputColor,
-                          const char* inputColor,
+                          const GrGLSLExpr4& inputColor,
                           const TransformedCoordsArray&,
                           const TextureSamplerArray&) SK_OVERRIDE;
 
@@ -344,7 +344,7 @@ void GrGLArithmeticEffect::emitCode(GrGLShaderBuilder* builder,
                                     const GrDrawEffect& drawEffect,
                                     EffectKey key,
                                     const char* outputColor,
-                                    const char* inputColor,
+                                    const GrGLSLExpr4& inputColor,
                                     const TransformedCoordsArray& coords,
                                     const TextureSamplerArray& samplers) {
 
@@ -362,24 +362,32 @@ void GrGLArithmeticEffect::emitCode(GrGLShaderBuilder* builder,
     SkASSERT(NULL != dstColor);
     fKUni = builder->addUniform(GrGLShaderBuilder::kFragment_Visibility,
                                 kVec4f_GrSLType, "k");
-    const char* kUni = builder->getUniformCStr(fKUni);
 
-    // We don't try to optimize for this case at all
-    if (NULL == inputColor) {
-        builder->fsCodeAppendf("\t\tconst vec4 src = vec4(1);\n");
+    GrGLSLExpr4 src;
+    if (gUseUnpremul) {
+        builder->fsCodeAppendf("\t\tvec4 src = %s;\n", inputColor.c_str());
+        builder->fsCodeAppendf("\t\tsrc.rgb = clamp(src.rgb / src.a, 0.0, 1.0);\n");
+        src = GrGLSLExpr4("src");
     } else {
-        builder->fsCodeAppendf("\t\tvec4 src = %s;\n", inputColor);
-        if (gUseUnpremul) {
-            builder->fsCodeAppendf("\t\tsrc.rgb = clamp(src.rgb / src.a, 0.0, 1.0);\n");
-        }
+        src = inputColor;
     }
 
-    builder->fsCodeAppendf("\t\tvec4 dst = %s;\n", dstColor);
+    GrGLSLExpr4 dst;
     if (gUseUnpremul) {
+        builder->fsCodeAppendf("\t\tvec4 dst = %s;\n", dstColor);
         builder->fsCodeAppendf("\t\tdst.rgb = clamp(dst.rgb / dst.a, 0.0, 1.0);\n");
+        dst = GrGLSLExpr4("dst");
+    } else {
+        dst = GrGLSLExpr4(dstColor);
     }
 
-    builder->fsCodeAppendf("\t\t%s = %s.x * src * dst + %s.y * src + %s.z * dst + %s.w;\n", outputColor, kUni, kUni, kUni, kUni);
+    GrGLSLExpr4 kExpr(builder->getUniformCStr(fKUni));
+    builder->fsCodeAppendf("\t\t%s = %s;\n", outputColor,
+                           (kExpr.x() * inputColor * dst +
+                            kExpr.y() * src +
+                            kExpr.z() * dst +
+                            kExpr.w()).c_str());
+
     builder->fsCodeAppendf("\t\t%s = clamp(%s, 0.0, 1.0);\n", outputColor, outputColor);
     if (gUseUnpremul) {
         builder->fsCodeAppendf("\t\t%s.rgb *= %s.a;\n", outputColor, outputColor);