Sk4px blit mask.

src/opts/SkBlitMask_opts.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 SkBlitMask_opts_DEFINED
+#define SkBlitMask_opts_DEFINED
+
+#include "Sk4px.h"
+
+namespace SK_OPTS_NS {
+
+static void blit_mask_d32_a8(SkPMColor* dst, size_t dstRB,
+                             const SkAlpha* mask, size_t maskRB,
+                             SkColor color, int w, int h) {
+    auto s = Sk4px::DupPMColor(SkPreMultiplyColor(color));
+
+    auto fn = [&](const Sk4px& d, const Sk4px& aa) {
+        //  = (s + d(1-sa))aa + d(1-aa)
+        //  = s*aa + d(1-sa*aa)
+        auto left  = s.approxMulDiv255(aa),
+             right = d.approxMulDiv255(left.alphas().inv());
+        return left + right;  // This does not overflow (exhaustively checked).
+    };
+
+    while (h --> 0) {
+        Sk4px::MapDstAlpha(w, dst, mask, fn);
+        dst  +=  dstRB / sizeof(*dst);
+        mask += maskRB / sizeof(*mask);
+    }
+}
+
+}  // SK_OPTS_NS
+
+#endif//SkBlitMask_opts_DEFINED