Templatize SkPMFloat to support both 1 and 255 biases.

src/core/SkPMFloat.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkPM_DEFINED
 #define SkPM_DEFINED
 
 #include "SkTypes.h"
 #include "SkColor.h"
 #include "SkColorPriv.h"
 #include "SkNx.h"
 
 // This file may be included multiple times by .cpp files with different flags, leading
 // to different definitions.  Usually that doesn't matter because it's all inlined, but
 // in Debug modes the compilers may not inline everything.  So wrap everything in an
 // anonymous namespace to give each includer their own silo of this code (or the linker
 // will probably pick one randomly for us, which is rarely correct).
 namespace {
 
 // A pre-multiplied color storing each component in the same order as SkPMColor,
-// but as a float in the range [0, 1].
+// but as a float in the range [0, kBias],
+// where kBias is either 1 (easier to work with) or 255 (sometimes a little faster).
+template <int kBias>
 class SkPMFloat : public Sk4f {
+    static_assert(kBias == 1 || kBias == 255, "");
 public:
     static SkPMFloat FromPMColor(SkPMColor c) { return SkPMFloat(c); }
     static SkPMFloat FromARGB(float a, float r, float g, float b) { return SkPMFloat(a,r,g,b); }
     static SkPMFloat FromOpaqueColor(SkColor c);  // Requires c's alpha == 0xFF.
 
     Sk4f alphas() const;  // argb -> aaaa, generally faster than the equivalent Sk4f(this->a()).
 
     // Uninitialized.
     SkPMFloat() {}
     explicit SkPMFloat(SkPMColor);
     SkPMFloat(float a, float r, float g, float b)
     #ifdef SK_PMCOLOR_IS_RGBA
         : INHERITED(r,g,b,a) {}
     #else
         : INHERITED(b,g,r,a) {}
     #endif
 
     SkPMFloat(const Sk4f& fs) : INHERITED(fs) {}
 
     float a() const { return this->kth<SK_A32_SHIFT / 8>(); }
     float r() const { return this->kth<SK_R32_SHIFT / 8>(); }
     float g() const { return this->kth<SK_G32_SHIFT / 8>(); }
     float b() const { return this->kth<SK_B32_SHIFT / 8>(); }
 
-    SkPMColor round() const;  // Rounds from [0.0f, 1.0f] to [0, 255], clamping if out of range.
+    SkPMColor round() const;  // Rounds from [0, kBias] to [0, 255], clamping if out of range.
 
     bool isValid() const {
-        return this->a() >= 0 && this->a() <= 1
+        return this->a() >= 0 && this->a() <= kBias
             && this->r() >= 0 && this->r() <= this->a()
             && this->g() >= 0 && this->g() <= this->a()
             && this->b() >= 0 && this->b() <= this->a();
     }
 
 private:
     typedef Sk4f INHERITED;
 };
 
 }  // namespace
 
 #ifdef SKNX_NO_SIMD
     // Platform implementations of SkPMFloat assume Sk4f uses SSE or NEON.  _none is generic.
     #include "../opts/SkPMFloat_none.h"
 #else
     #if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
         #include "../opts/SkPMFloat_sse.h"
     #elif defined(SK_ARM_HAS_NEON)
         #include "../opts/SkPMFloat_neon.h"
     #else
         #include "../opts/SkPMFloat_none.h"
     #endif
 #endif
 
 #endif//SkPM_DEFINED