Use texture barriers to read directly from the RT

include/gpu/GrXferProcessor.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrXferProcessor_DEFINED
 #define GrXferProcessor_DEFINED
 
@@ skipping 30 matching lines @@
     kIConstA_GrBlendCoeff, //<! one minus constant color alpha
     kS2C_GrBlendCoeff,
     kIS2C_GrBlendCoeff,
     kS2A_GrBlendCoeff,
     kIS2A_GrBlendCoeff,
 
     kTotalGrBlendCoeffCount
 };
 
 /**
+ * Barriers for blending. When a shader reads the dst directly, an Xfer barrier is sometimes
+ * required after a pixel has been written, before it can be safely read again.
+ */
+enum GrXferBarrierType {
+    kTexture_GrXferBarrierType, //<! Required when a shader reads and renders to the same texture.
+};
+
+/**
  * GrXferProcessor is responsible for implementing the xfer mode that blends the src color and dst
  * color. It does this by emitting fragment shader code and controlling the fixed-function blend
  * state. The inputs to its shader code are the final computed src color and fractional pixel
  * coverage. The GrXferProcessor's shader code writes the fragment shader output color that goes
  * into the fixed-function blend. When dual-source blending is available, it may also write a
  * seconday fragment shader output color. When allowed by the backend API, the GrXferProcessor may
  * read the destination color. The GrXferProcessor is responsible for setting the blend coefficients
  * and blend constant color.
  *
  * A GrXferProcessor is never installed directly into our draw state, but instead is created from a
@@ skipping 56 matching lines @@
      * overrideColor will be the required value that should be passed into the XP. 
      * A caller who calls this function on a XP is required to honor the returned OptFlags
      * and color values for its draw.
      */
     virtual OptFlags getOptimizations(const GrProcOptInfo& colorPOI,
                                       const GrProcOptInfo& coveragePOI,
                                       bool doesStencilWrite,
                                       GrColor* overrideColor,
                                       const GrDrawTargetCaps& caps) = 0;
 
+    /**
+     * Returns whether this XP will require an Xfer barrier on the given rt. If true, outBarrierType
+     * is updated to contain the type of barrier needed.
+     */
+    virtual bool willNeedXferBarrier(const GrRenderTarget* rt,
+                                     const GrDrawTargetCaps& caps,
+                                     GrXferBarrierType* outBarrierType) const;
+
     struct BlendInfo {
         GrBlendCoeff fSrcBlend;
         GrBlendCoeff fDstBlend;
         GrColor      fBlendConstant;
         bool         fWriteColor;
     };
 
     void getBlendInfo(BlendInfo* blendInfo) const {
         blendInfo->fSrcBlend = kOne_GrBlendCoeff;
         blendInfo->fDstBlend = kZero_GrBlendCoeff;
@@ skipping 184 matching lines @@
     enum {
         kIllegalXPFClassID = 0,
     };
     static int32_t gCurrXPFClassID;
 
     typedef GrProgramElement INHERITED;
 };
 
 #endif