Implement Porter Duff XP with a blend table

src/gpu/GrBlend.h

Index: src/gpu/GrBlend.h
diff --git a/src/gpu/GrBlend.h b/src/gpu/GrBlend.h
index e5b8993377a4ae5baba5c61ad8bb373424d42f19..7021902765bd07024cbe4b8c0d88961c87504908 100644
--- a/src/gpu/GrBlend.h
+++ b/src/gpu/GrBlend.h
@@ -7,13 +7,22 @@
  */
 
 #include "GrTypes.h"
-#include "GrColor.h"
+#include "SkTLogic.h"
 #include "GrXferProcessor.h"
 
 #ifndef GrBlend_DEFINED
 #define GrBlend_DEFINED
 
-static inline bool GrBlendCoeffRefsSrc(GrBlendCoeff coeff) {
+template<GrBlendCoeff Coeff>
+struct GrTBlendCoeffRefsSrc : SkTBool<kSC_GrBlendCoeff == Coeff ||
+                                      kISC_GrBlendCoeff == Coeff ||
+                                      kSA_GrBlendCoeff == Coeff ||
+                                      kISA_GrBlendCoeff == Coeff> {};
+
+#define GR_BLEND_COEFF_REFS_SRC(COEFF) \
+    GrTBlendCoeffRefsSrc<COEFF>::value
+
+inline bool GrBlendCoeffRefsSrc(GrBlendCoeff coeff) {
     switch (coeff) {
         case kSC_GrBlendCoeff:
         case kISC_GrBlendCoeff:
@@ -25,7 +34,17 @@ static inline bool GrBlendCoeffRefsSrc(GrBlendCoeff coeff) {
     }
 }
 
-static inline bool GrBlendCoeffRefsDst(GrBlendCoeff coeff) {
+
+template<GrBlendCoeff Coeff>
+struct GrTBlendCoeffRefsDst : SkTBool<kDC_GrBlendCoeff == Coeff ||
+                                      kIDC_GrBlendCoeff == Coeff ||
+                                      kDA_GrBlendCoeff == Coeff ||
+                                      kIDA_GrBlendCoeff == Coeff> {};
+
+#define GR_BLEND_COEFF_REFS_DST(COEFF) \
+    GrTBlendCoeffRefsDst<COEFF>::value
+
+inline bool GrBlendCoeffRefsDst(GrBlendCoeff coeff) {
     switch (coeff) {
         case kDC_GrBlendCoeff:
         case kIDC_GrBlendCoeff:
@@ -37,10 +56,100 @@ static inline bool GrBlendCoeffRefsDst(GrBlendCoeff coeff) {
     }
 }
 
-GrColor GrSimplifyBlend(GrBlendCoeff* srcCoeff,
-                        GrBlendCoeff* dstCoeff,
-                        GrColor srcColor, uint32_t srcCompFlags,
-                        GrColor dstColor, uint32_t dstCompFlags,
-                        GrColor constantColor);
+
+template<GrBlendCoeff Coeff>
+struct GrTBlendCoeffRefsSrc2 : SkTBool<kS2C_GrBlendCoeff == Coeff ||
+                                       kIS2C_GrBlendCoeff == Coeff ||
+                                       kS2A_GrBlendCoeff == Coeff ||
+                                       kIS2A_GrBlendCoeff == Coeff> {};
+
+#define GR_BLEND_COEFF_REFS_SRC2(COEFF) \
+    GrTBlendCoeffRefsSrc2<COEFF>::value
+
+inline bool GrBlendCoeffRefsSrc2(GrBlendCoeff coeff) {
+    switch (coeff) {
+        case kS2C_GrBlendCoeff:
+        case kIS2C_GrBlendCoeff:
+        case kS2A_GrBlendCoeff:
+        case kIS2A_GrBlendCoeff:
+            return true;
+        default:
+            return false;
+    }
+}
+
+
+template<GrBlendCoeff SrcCoeff, GrBlendCoeff DstCoeff>
+struct GrTBlendCoeffsUseSrcColor : SkTBool<kZero_GrBlendCoeff != SrcCoeff ||
+                                           GR_BLEND_COEFF_REFS_SRC(DstCoeff)> {};
+
+#define GR_BLEND_COEFFS_USE_SRC_COLOR(SRC_COEFF, DST_COEFF) \
+    GrTBlendCoeffsUseSrcColor<SRC_COEFF, DST_COEFF>::value
+
+
+template<GrBlendCoeff SrcCoeff, GrBlendCoeff DstCoeff>
+struct GrTBlendCoeffsUseDstColor : SkTBool<GR_BLEND_COEFF_REFS_DST(SrcCoeff) ||
+                                           kZero_GrBlendCoeff != DstCoeff> {};
+
+#define GR_BLEND_COEFFS_USE_DST_COLOR(SRC_COEFF, DST_COEFF) \
+    GrTBlendCoeffsUseDstColor<SRC_COEFF, DST_COEFF>::value
+
+
+template<GrBlendEquation Equation>
+struct GrTBlendEquationIsAdvanced : SkTBool<Equation >= kFirstAdvancedGrBlendEquation> {};
+
+#define GR_BLEND_EQUATION_IS_ADVANCED(EQUATION) \
+    GrTBlendEquationIsAdvanced<EQUATION>::value
+
+inline bool GrBlendEquationIsAdvanced(GrBlendEquation equation) {
+    return equation >= kFirstAdvancedGrBlendEquation;
+}
+
+
+template<GrBlendEquation BlendEquation, GrBlendCoeff SrcCoeff, GrBlendCoeff DstCoeff>
+struct GrTBlendModifiesDst : SkTBool<(kAdd_GrBlendEquation != BlendEquation &&
+                                      kReverseSubtract_GrBlendEquation != BlendEquation) ||
+                                     kZero_GrBlendCoeff != SrcCoeff ||
+                                     kOne_GrBlendCoeff != DstCoeff> {};
+
+#define GR_BLEND_MODIFIES_DST(EQUATION, SRC_COEFF, DST_COEFF) \
+    GrTBlendModifiesDst<EQUATION, SRC_COEFF, DST_COEFF>::value
+
+
+/**
+ * Advanced blend equations can always tweak alpha for coverage. (See GrCustomXfermode.cpp)
+ *
+ * For "add" and "reverse subtract" the blend equation with f=coverage is:
+ *
+ *   D' = f * (S * srcCoeff + D * dstCoeff) + (1-f) * D
+ *      = f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
+ *
+ * (Let srcCoeff be negative for reverse subtract.) We can tweak alpha for coverage when the
+ * following relationship holds:
+ *
+ *   (f*S) * srcCoeff' + D * dstCoeff' == f * S * srcCoeff + D * (f * dstCoeff + (1 - f))
+ *
+ * (Where srcCoeff' and dstCoeff' have any reference to S pre-multiplied by f.)
+ *
+ * It's easy to see this works for the src term as long as srcCoeff' == srcCoeff (meaning srcCoeff
+ * does not reference S). For the dst term, this will work as long as the following is true:
+ *|
+ *   dstCoeff' == f * dstCoeff + (1 - f)
+ *   dstCoeff' == 1 - f * (1 - dstCoeff)
+ *
+ * By inspection we can see this will work as long as dstCoeff has a 1, and any other term in
+ * dstCoeff references S.
+ */
+template<GrBlendEquation Equation, GrBlendCoeff SrcCoeff, GrBlendCoeff DstCoeff>
+struct GrTBlendCanTweakAlphaForCoverage : SkTBool<GR_BLEND_EQUATION_IS_ADVANCED(Equation) ||
+                                                  ((kAdd_GrBlendEquation == Equation ||
+                                                    kReverseSubtract_GrBlendEquation == Equation) &&
+                                                   !GR_BLEND_COEFF_REFS_SRC(SrcCoeff) &&
+                                                   (kOne_GrBlendCoeff == DstCoeff ||
+                                                    kISC_GrBlendCoeff == DstCoeff ||
+                                                    kISA_GrBlendCoeff == DstCoeff))> {};
+
+#define GR_BLEND_CAN_TWEAK_ALPHA_FOR_COVERAGE(EQUATION, SRC_COEFF, DST_COEFF) \
+    GrTBlendCanTweakAlphaForCoverage<EQUATION, SRC_COEFF, DST_COEFF>::value
 
 #endif