Add mixed samples support to XPs

src/gpu/gl/GrGLXferProcessor.cpp

Index: src/gpu/gl/GrGLXferProcessor.cpp
diff --git a/src/gpu/gl/GrGLXferProcessor.cpp b/src/gpu/gl/GrGLXferProcessor.cpp
index b707a3f632047d4892518d42f55f1b2454bcc83f..acce085790fce59bfe73871090a28be50a5645f0 100644
--- a/src/gpu/gl/GrGLXferProcessor.cpp
+++ b/src/gpu/gl/GrGLXferProcessor.cpp
@@ -12,47 +12,54 @@
 #include "gl/builders/GrGLProgramBuilder.h"
 
 void GrGLXferProcessor::emitCode(const EmitArgs& args) {
-    if (args.fXP.getDstTexture()) {
-        bool topDown = kTopLeft_GrSurfaceOrigin == args.fXP.getDstTexture()->origin();
-
+    if (args.fXP.willReadDstColor()) {
         GrGLXPFragmentBuilder* fsBuilder = args.fPB->getFragmentShaderBuilder();
 
-        if (args.fXP.readsCoverage()) {
-            // We don't think any shaders actually output negative coverage, but just as a safety
-            // check for floating point precision errors we compare with <= here
-            fsBuilder->codeAppendf("if (all(lessThanEqual(%s, vec4(0)))) {"
-                                   "    discard;"
-                                   "}", args.fInputCoverage);
-        }
+        if (args.fXP.getDstTexture()) {
+            bool topDown = kTopLeft_GrSurfaceOrigin == args.fXP.getDstTexture()->origin();
+
+            if (args.fXP.readsCoverage()) {
+                // We don't think any shaders actually output negative coverage, but just as a
+                // safety check for floating point precision errors we compare with <= here
+                fsBuilder->codeAppendf("if (all(lessThanEqual(%s, vec4(0)))) {"
+                                       "    discard;"
+                                       "}", args.fInputCoverage);
+            }
 
-        const char* dstColor = fsBuilder->dstColor();
+            const char* dstColor = fsBuilder->dstColor();
 
-        const char* dstTopLeftName;
-        const char* dstCoordScaleName;
+            const char* dstTopLeftName;
+            const char* dstCoordScaleName;
 
-        fDstTopLeftUni = args.fPB->addUniform(GrGLProgramBuilder::kFragment_Visibility,
-                                              kVec2f_GrSLType,
-                                              kDefault_GrSLPrecision,
-                                              "DstTextureUpperLeft",
-                                              &dstTopLeftName);
-        fDstScaleUni = args.fPB->addUniform(GrGLProgramBuilder::kFragment_Visibility,
-                                            kVec2f_GrSLType,
-                                            kDefault_GrSLPrecision,
-                                            "DstTextureCoordScale",
-                                            &dstCoordScaleName);
-        const char* fragPos = fsBuilder->fragmentPosition();
+            fDstTopLeftUni = args.fPB->addUniform(GrGLProgramBuilder::kFragment_Visibility,
+                                                  kVec2f_GrSLType,
+                                                  kDefault_GrSLPrecision,
+                                                  "DstTextureUpperLeft",
+                                                  &dstTopLeftName);
+            fDstScaleUni = args.fPB->addUniform(GrGLProgramBuilder::kFragment_Visibility,
+                                                kVec2f_GrSLType,
+                                                kDefault_GrSLPrecision,
+                                                "DstTextureCoordScale",
+                                                &dstCoordScaleName);
+            const char* fragPos = fsBuilder->fragmentPosition();
 
-        fsBuilder->codeAppend("// Read color from copy of the destination.\n");
-        fsBuilder->codeAppendf("vec2 _dstTexCoord = (%s.xy - %s) * %s;",
-                               fragPos, dstTopLeftName, dstCoordScaleName);
+            fsBuilder->codeAppend("// Read color from copy of the destination.\n");
+            fsBuilder->codeAppendf("vec2 _dstTexCoord = (%s.xy - %s) * %s;",
+                                   fragPos, dstTopLeftName, dstCoordScaleName);
 
-        if (!topDown) {
-            fsBuilder->codeAppend("_dstTexCoord.y = 1.0 - _dstTexCoord.y;");
+            if (!topDown) {
+                fsBuilder->codeAppend("_dstTexCoord.y = 1.0 - _dstTexCoord.y;");
+            }
+
+            fsBuilder->codeAppendf("vec4 %s = ", dstColor);
+            fsBuilder->appendTextureLookup(args.fSamplers[0], "_dstTexCoord", kVec2f_GrSLType);
+            fsBuilder->codeAppend(";");
         }
 
-        fsBuilder->codeAppendf("vec4 %s = ", dstColor);
-        fsBuilder->appendTextureLookup(args.fSamplers[0], "_dstTexCoord", kVec2f_GrSLType);
-        fsBuilder->codeAppend(";");
+        if (args.fXP.dstReadUsesMixedSamples()) {
+            // Output 1.0, which the hardware will modulate by sample-based coverage before blend.
+            fsBuilder->codeAppendf("%s = vec4(1.0);", args.fOutputSecondary);
+        }
     }
 
     this->onEmitCode(args);