Perform coverage blend with the dst in the shader when using a dst-reading xfermode.

src/gpu/gl/GrGLProgramDesc.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGLProgramDesc_DEFINED
 #define GrGLProgramDesc_DEFINED
 
@@ skipping 47 matching lines @@
 private:
     // Specifies where the initial color comes from before the stages are applied.
     enum ColorInput {
         kSolidWhite_ColorInput,
         kTransBlack_ColorInput,
         kAttribute_ColorInput,
         kUniform_ColorInput,
 
         kColorInputCnt
     };
-    // Dual-src blending makes use of a secondary output color that can be
-    // used as a per-pixel blend coefficient. This controls whether a
-    // secondary source is output and what value it holds.
-    enum DualSrcOutput {
-        kNone_DualSrcOutput,
-        kCoverage_DualSrcOutput,
-        kCoverageISA_DualSrcOutput,
-        kCoverageISC_DualSrcOutput,
 
-        kDualSrcOutputCnt
+    enum CoverageOutput {
+        // modulate color and coverage, write result as the color output.
+        kModulate_CoverageOutput,
+        // Writes color*coverage as the primary color output and also writes coverage as the
+        // secondary output. Only set if dual source blending is supported.
+        kSecondaryCoverage_CoverageOutput,
+        // Writes color*coverage as the primary color output and also writes coverage * (1 - colorA)
+        // as the secondary output. Only set if dual source blending is supported.
+        kSecondaryCoverageISA_CoverageOutput,
+        // Writes color*coverage as the primary color output and also writes coverage *
+        // (1 - colorRGB) as the secondary output. Only set if dual source blending is supported.
+        kSecondaryCoverageISC_CoverageOutput,
+        // Combines the coverage, dst, and color as coverage * color + (1 - coverage) * dst. This
+        // can only be set if fDstRead is set.
+        kCombineWithDst_CoverageOutput,
+
+        kCoverageOutputCnt
     };
 
+    static bool CoverageOutputUsesSecondaryOutput(CoverageOutput co) {
+        switch (co) {
+            case kSecondaryCoverage_CoverageOutput: //  fallthru
+            case kSecondaryCoverageISA_CoverageOutput:
+            case kSecondaryCoverageISC_CoverageOutput:
+                return true;
+            default:
+                return false;
+        }
+    }
+
     /** Non-zero if this stage has an effect */
     GrGLEffect::EffectKey       fEffectKeys[GrDrawState::kNumStages];
 
     // To enable experimental geometry shader code (not for use in
     // production)
 #if GR_GL_EXPERIMENTAL_GS
     bool                     fExperimentalGS;
 #endif
 
     GrGLShaderBuilder::DstReadKey fDstRead;             // set by GrGLShaderBuilder if there
                                                         // are effects that must read the dst.
                                                         // Otherwise, 0.
 
     // should the FS discard if the coverage is zero (to avoid stencil manipulation)
     SkBool8                     fDiscardIfZeroCoverage;
 
     uint8_t                     fColorInput;            // casts to enum ColorInput
     uint8_t                     fCoverageInput;         // casts to enum ColorInput
-    uint8_t                     fDualSrcOutput;         // casts to enum DualSrcOutput
+    uint8_t                     fCoverageOutput;        // casts to enum CoverageOutput
 
     int8_t                      fFirstCoverageStage;
     SkBool8                     fEmitsPointSize;
     uint8_t                     fColorFilterXfermode;   // casts to enum SkXfermode::Mode
 
     int8_t                      fPositionAttributeIndex;
     int8_t                      fLocalCoordAttributeIndex;
     int8_t                      fColorAttributeIndex;
     int8_t                      fCoverageAttributeIndex;
 
     // GrGLProgram and GrGLShaderBuilder read the private fields to generate code. TODO: Move all
     // code generation to GrGLShaderBuilder (and maybe add getters rather than friending).
     friend class GrGLProgram;
     friend class GrGLShaderBuilder;
 };
 
 #endif