Add a hook for CPU-optimized sRGB-sRGB srcover.

src/opts/SkBlend_opts.h

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkBlend_opts_DEFINED
+#define SkBlend_opts_DEFINED
+
+namespace SK_OPTS_NS {
+
+#if 0
+
+#else
+
+    static inline void srcover_srgb_srgb_1(uint32_t* dst, uint32_t src) {
+        switch (src >> 24) {

[f(malita), 2016/05/02 18:19:27]

Should dst/src be SkPMColor to make the swizzle explicit?

Maybe use SK_A32_SHIFT, SK_AlphaTRANSPARENT, SK_AlphaOPAQUE?

+            case 0x00:             return; 
+            case 0xff: *dst = src; return;
+        }
+
+        Sk4f d = SkNx_cast<float>(Sk4b::Load( dst)),
+             s = SkNx_cast<float>(Sk4b::Load(&src));
+
+        // Approximate sRGB gamma as 2.0.
+        Sk4f d_sq = d*d,
+             s_sq = s*s;
+        d = Sk4f{d_sq[0], d_sq[1], d_sq[2], d[3]};
+        s = Sk4f{s_sq[0], s_sq[1], s_sq[2], s[3]};
+
+        // SrcOver.
+        Sk4f invA = 1.0f - s[3]*(1/255.0f);
+        d = s + d * invA;
+
+        // Re-apply approximate sRGB gamma.
+        Sk4f d_sqrt = d.sqrt();
+        d = Sk4f{d_sqrt[0], d_sqrt[1], d_sqrt[2], d[3]};
+
+        SkNx_cast<uint8_t>(d).store(dst);
+    }
+
+    static inline void srcover_srgb_srgb(uint32_t* dst, const uint32_t* const src, int ndst, const int nsrc) {
+        while (ndst > 0) {
+            int n = SkTMin(ndst, nsrc);
+
+            for (int i = 0; i < n; i++) {
+                srcover_srgb_srgb_1(dst++, src[i]);
+            }
+            ndst -= n;
+        }
+    }
+    
+#endif
+
+}  // namespace SK_OPTS_NS
+
+#endif//SkBlend_opts_DEFINED