Replace fWillReadFragmentPosition with a bitfield

include/gpu/GrProcessor.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrProcessor_DEFINED
 #define GrProcessor_DEFINED
 
@@ skipping 61 matching lines @@
 
     int numTextures() const { return fTextureAccesses.count(); }
 
     /** Returns the access pattern for the texture at index. index must be valid according to
         numTextures(). */
     const GrTextureAccess& textureAccess(int index) const { return *fTextureAccesses[index]; }
 
     /** Shortcut for textureAccess(index).texture(); */
     GrTexture* texture(int index) const { return this->textureAccess(index).getTexture(); }
 
-    /** Will this processor read the fragment position? */
-    bool willReadFragmentPosition() const { return fWillReadFragmentPosition; }
+    /**
+     * Platform specific built-in state that a processor can include in the fragment shader. A
+     * processor must enable this state before using it.
+     */
+    enum BuiltInState {
+        kNone_BuiltInState              = 0,
+        kFragmentPosition_BuiltInState  = 1
+    };
+
+    GR_DECL_BITFIELD_OPS_FRIENDS(BuiltInState);
+
+    BuiltInState builtInState() const { return fBuiltInState; }
 
     void* operator new(size_t size);
     void operator delete(void* target);
 
     void* operator new(size_t size, void* placement) {
         return ::operator new(size, placement);
     }
     void operator delete(void* target, void* placement) {
         ::operator delete(target, placement);
     }
 
     /**
       * Helper for down-casting to a GrProcessor subclass
       */
     template <typename T> const T& cast() const { return *static_cast<const T*>(this); }
 
     uint32_t classID() const { SkASSERT(kIllegalProcessorClassID != fClassID); return fClassID; }
 
 protected:
-    GrProcessor() : fClassID(kIllegalProcessorClassID), fWillReadFragmentPosition(false) {}
+    GrProcessor() : fClassID(kIllegalProcessorClassID), fBuiltInState(kNone_BuiltInState) {}
 
     /**
      * Subclasses call this from their constructor to register GrTextureAccesses. The processor
      * subclass manages the lifetime of the accesses (this function only stores a pointer). The
      * GrTextureAccess is typically a member field of the GrProcessor subclass. This must only be
      * called from the constructor because GrProcessors are immutable.
      */
     virtual void addTextureAccess(const GrTextureAccess* textureAccess);
 
     bool hasSameTextureAccesses(const GrProcessor&) const;
 
-    /**
-     * If the prcoessor will generate a backend-specific processor that will read the fragment
-     * position in the FS then it must call this method from its constructor. Otherwise, the
-     * request to access the fragment position will be denied.
-     */
-    void setWillReadFragmentPosition() { fWillReadFragmentPosition = true; }
+    void enableBuiltInState(BuiltInState state) { fBuiltInState |= state; }

[joshualitt, 2016/02/25 19:21:29]

This is more a question for Brian / Robert, but do we want to hide the fact that
the implementation is a bit field? I kind of like setWillReadFragmentPosition /
willReadFragmentPosition, though I also like that compose shader can just grab
the whole bitfield of its children.  Maybe retain the old willRead/setWillRead
calls, but also allow access to the BuiltInState enum?  I don't really have a
strong preference.

[Chris Dalton, 2016/02/25 19:45:30]

So, willRead isn't very needed anymore on this class. GrPipeline in this change
still keeps "willReadFragPosition", and that's where everybody else asks this
question AFAIK.

[Chris Dalton, 2016/02/25 20:38:37]

Also, FWIW, this feels pretty similar to "fsBuilder->enableFeature(kMy_Feature)"

[bsalomon, 2016/02/25 21:11:03]

I find the setWillFoo() a bit more readable than enableFeature(kFoo). It doesn't
feel like these are inherently related, just happen to be sharing a bitfield for
implementation.

WRT children FPs, I really hope we replace the current mess of pulling state
into the parent with a system that iterates over the DAG externally.

[Chris Dalton, 2016/02/25 22:24:24]

Ok, brought back "setWillFoo()".

For children FPs, I added a combineBuiltInState() method. We can gladly remove
it once child FPs change.

 
     template <typename PROC_SUBCLASS> void initClassID() {
          static uint32_t kClassID = GenClassID();
          fClassID = kClassID;
     }
 
     uint32_t fClassID;
     SkSTArray<4, const GrTextureAccess*, true>   fTextureAccesses;
 
 private:
     static uint32_t GenClassID() {
         // fCurrProcessorClassID has been initialized to kIllegalProcessorClassID. The
         // atomic inc returns the old value not the incremented value. So we add
         // 1 to the returned value.
         uint32_t id = static_cast<uint32_t>(sk_atomic_inc(&gCurrProcessorClassID)) + 1;
         if (!id) {
             SkFAIL("This should never wrap as it should only be called once for each GrProcessor "
                    "subclass.");
         }
         return id;
     }
 
     enum {
         kIllegalProcessorClassID = 0,
     };
     static int32_t gCurrProcessorClassID;
 
-    bool                                         fWillReadFragmentPosition;
+    BuiltInState fBuiltInState;
 
     typedef GrProgramElement INHERITED;
 };
 
+GR_MAKE_BITFIELD_OPS(GrProcessor::BuiltInState);
+
 #endif