Optimize Gaussian convolution fragment processor.

This FP now uses an explicit return statement instead of sk_OutColor.
Additionally, simplified the generated code by removing a temp variable
that did nothing (coordSampled), and by removing a needless addition at
the end of the loop.

It looks like coordSampled was useful when it was introduced at
http://review.skia.org/20465, but it was later rendered useless.

Change-Id: I8c549b16b4d422d7faeab48e2087f168ad5788b5
Bug: skia:10549
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/344156
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>

Author: johnstiles-google

src/gpu/effects/GrGaussianConvolutionFragmentProcessor.cpp

@@ -57,24 +57,20 @@ void GrGaussianConvolutionFragmentProcessor::Impl::emitCode(EmitArgs& args) {
 
     GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
 
-    fragBuilder->codeAppendf("%s = half4(0);", args.fOutputColor);
+    fragBuilder->codeAppendf("half4 color = half4(0);");
 
     fragBuilder->codeAppendf("float2 coord = %s - %d.0 * %s;", args.fSampleCoord, ce.fRadius, inc);
-    fragBuilder->codeAppend("float2 coordSampled = half2(0);");
 
     // Manually unroll loop because some drivers don't; yields 20-30% speedup.
-    static constexpr const char* kVecSuffix[4] = {".x", ".y", ".z", ".w"};
     for (int i = 0; i < width; i++) {
-        SkString kernelIndex;
-        kernelIndex.printf("%s[%d]", kernel, i/4);
-        kernelIndex.append(kVecSuffix[i & 0x3]);
-
-        fragBuilder->codeAppend("coordSampled = coord;");
-        auto sample = this->invokeChild(0, args, "coordSampled");
-        fragBuilder->codeAppendf("%s += %s", args.fOutputColor, sample.c_str());
-        fragBuilder->codeAppendf(" * %s;", kernelIndex.c_str());
-        fragBuilder->codeAppendf("coord += %s;", inc);
+        auto sample = this->invokeChild(/*childIndex=*/0, args, "coord");
+        if (i != 0) {
+            fragBuilder->codeAppendf("coord += %s;", inc);
+        }
+        fragBuilder->codeAppendf("color += %s * %s[%d][%d];",
+                                 sample.c_str(), kernel, i / 4, i & 0x3);
     }
+    fragBuilder->codeAppendf("return color;");
 }
 
 void GrGaussianConvolutionFragmentProcessor::Impl::onSetData(const GrGLSLProgramDataManager& pdman,

src/gpu/effects/GrGaussianConvolutionFragmentProcessor.h

@@ -39,6 +39,7 @@ class GrGaussianConvolutionFragmentProcessor : public GrFragmentProcessor {
                                                      const GrCaps&);
 
     const char* name() const override { return "GaussianConvolution"; }
+    bool usesExplicitReturn() const override { return true; }
 
     std::unique_ptr<GrFragmentProcessor> clone() const override {
         return std::unique_ptr<GrFragmentProcessor>(