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

Herb's really starting to get serious about tweaking this, which becomes
a lot easier when you've got SkOpts' runtime CPU detection. We should be
able to optimize this usefully for SSSE3, SSE4.1, AVX, AVX2, or NEON.
(We can of course implement a subset.)

This function takes two counts to give us flexibility to write src patterns:
   nsrc >= ndst -> the usual srcover function
   nsrc <  ndst -> repeat src until it fills dst
   nsrc << ndst -> possibly preprocess src into registers
   nsrc == 1    -> equivalent of blitrow_color32, srcover_1, etc.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1939783003

Review-Url: https://codereview.chromium.org/1939783003

1 files changed, 58 insertions, 0 deletions

diff --git a/src/opts/SkBlend_opts.h b/src/opts/SkBlend_opts.h
new file mode 100644
index 0000000000..a1067407be
--- /dev/null
+++ b/src/opts/SkBlend_opts.h
@@ -0,0 +1,58 @@
+/*
+ * 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) {
+            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