Use noperspective interpolation for 2D draws

src/gpu/glsl/GrGLSLVarying.h

Index: src/gpu/glsl/GrGLSLVarying.h
diff --git a/src/gpu/glsl/GrGLSLVarying.h b/src/gpu/glsl/GrGLSLVarying.h
index 22431978c859666d57d33dcc0619c51694420c0e..8697ebb129e7ded8ba709d1ff45f92e79520585c 100644
--- a/src/gpu/glsl/GrGLSLVarying.h
+++ b/src/gpu/glsl/GrGLSLVarying.h
@@ -71,18 +71,25 @@ static const int kVaryingsPerBlock = 8;
 class GrGLSLVaryingHandler {
 public:
     explicit GrGLSLVaryingHandler(GrGLSLProgramBuilder* program)
-        : fVertexInputs(kVaryingsPerBlock)
+        : fVaryings(kVaryingsPerBlock)
+        , fVertexInputs(kVaryingsPerBlock)
         , fVertexOutputs(kVaryingsPerBlock)
         , fGeomInputs(kVaryingsPerBlock)
         , fGeomOutputs(kVaryingsPerBlock)
         , fFragInputs(kVaryingsPerBlock)
         , fFragOutputs(kVaryingsPerBlock)
-        , fProgramBuilder(program) {}
+        , fProgramBuilder(program)
+        , fDefaultInterpolationModifier(nullptr) {}
 
     virtual ~GrGLSLVaryingHandler() {}
 
-    typedef GrTAllocator<GrGLSLShaderVar> VarArray;
-    typedef GrGLSLProgramDataManager::VaryingHandle VaryingHandle;
+    /*
+     * Notifies the varying handler that this shader will never emit geometry in perspective and
+     * therefore does not require perspective-correct interpolation. When supported, this allows
+     * varyings to use the "noperspective" keyword, which means the GPU can use cheaper math for
+     * interpolation.
+     */
+    void setNoPerspective();
 
     /*
      * addVarying allows fine grained control for setting up varyings between stages. Calling this
@@ -93,18 +100,36 @@ public:
      * TODO convert most uses of addVarying to addPassThroughAttribute
      */
     void addVarying(const char* name,
-                    GrGLSLVarying*,
-                    GrSLPrecision precision = kDefault_GrSLPrecision);
+                    GrGLSLVarying* varying,
+                    GrSLPrecision precision = kDefault_GrSLPrecision) {
+        SkASSERT(GrSLTypeIsFloatType(varying->type())); // Integers must use addFlatVarying.
+        this->internalAddVarying(name, varying, precision, false /*flat*/);
+    }
+
+    /*
+     * addFlatVarying sets up a varying whose value is constant across every fragment. The graphics
+     * pipeline will pull its value from the final vertex of the draw primitive (provoking vertex).
+     * Flat interpolation is not always supported and the user must check the caps before using.
+     * TODO: Some platforms can change the provoking vertex. Should we be resetting this knob?
+     */
+    void addFlatVarying(const char* name,
+                        GrGLSLVarying* varying,
+                        GrSLPrecision precision = kDefault_GrSLPrecision) {
+        this->internalAddVarying(name, varying, precision, true /*flat*/);
+    }
 
     /*
-     * This call can be used by GP to pass an attribute through all shaders directly to 'output' in
-     * the fragment shader.  Though this call effects both the vertex shader and fragment shader,
-     * it expects 'output' to be defined in the fragment shader before this call is made. If there
+     * The GP can use these calls to pass an attribute through all shaders directly to 'output' in
+     * the fragment shader.  Though these calls affect both the vertex shader and fragment shader,
+     * they expect 'output' to be defined in the fragment shader before the call is made. If there
      * is a geometry shader, we will simply take the value of the varying from the first vertex and
      * that will be set as the output varying for all emitted vertices.
-     * TODO it might be nicer behavior to have a flag to declare output inside this call
+     * TODO it might be nicer behavior to have a flag to declare output inside these calls
      */
-    void addPassThroughAttribute(const GrGeometryProcessor::Attribute*, const char* output);
+    void addPassThroughAttribute(const GrGeometryProcessor::Attribute*, const char* output,
+                                 GrSLPrecision = kDefault_GrSLPrecision);
+    void addFlatPassThroughAttribute(const GrGeometryProcessor::Attribute*, const char* output,
+                                     GrSLPrecision = kDefault_GrSLPrecision);
 
     void emitAttributes(const GrGeometryProcessor& gp);
 
@@ -115,21 +140,36 @@ public:
     void getVertexDecls(SkString* inputDecls, SkString* outputDecls) const;
     void getGeomDecls(SkString* inputDecls, SkString* outputDecls) const;
     void getFragDecls(SkString* inputDecls, SkString* outputDecls) const;
+
 protected:
-    VarArray fVertexInputs;
-    VarArray fVertexOutputs;
-    VarArray fGeomInputs;
-    VarArray fGeomOutputs;
-    VarArray fFragInputs;
-    VarArray fFragOutputs;
+    struct VaryingInfo {
+        GrSLType         fType;
+        GrSLPrecision    fPrecision;
+        bool             fIsFlat;
+        SkString         fName;

[egdaniel, 2016/02/12 18:53:06]

I think using fName is kind of confusing since it really just means fVsOut. I
say we just change the name back to that, but leave the changed logic/uses the
same.

[Chris Dalton, 2016/02/12 19:35:03]

Done.

+        SkString         fGsOut;
+        GrShaderFlags    fVisibility;
+    };
+
+    typedef GrTAllocator<VaryingInfo> VaryingList;
+    typedef GrTAllocator<GrGLSLShaderVar> VarArray;
+    typedef GrGLSLProgramDataManager::VaryingHandle VaryingHandle;
+
+    VaryingList    fVaryings;
+    VarArray       fVertexInputs;
+    VarArray       fVertexOutputs;
+    VarArray       fGeomInputs;
+    VarArray       fGeomOutputs;
+    VarArray       fFragInputs;
+    VarArray       fFragOutputs;
 
     // This is not owned by the class
     GrGLSLProgramBuilder* fProgramBuilder;
 
 private:
-    void addVertexVarying(const char* name, GrSLPrecision precision, GrGLSLVarying* v);
-    void addGeomVarying(const char* name, GrSLPrecision precision, GrGLSLVarying* v);
-    void addFragVarying(GrSLPrecision precision, GrGLSLVarying* v);
+    void internalAddVarying(const char* name, GrGLSLVarying*, GrSLPrecision, bool flat);
+    void writePassThroughAttribute(const GrGeometryProcessor::Attribute*, const char* output,
+                                   const GrGLSLVarying&);
 
     void addAttribute(const GrShaderVar& var);
 
@@ -138,6 +178,8 @@ private:
     // helper function for get*Decls
     void appendDecls(const VarArray& vars, SkString* out) const;
 
+    const char* fDefaultInterpolationModifier;
+
     friend class GrGLSLProgramBuilder;
 };