Optimized premultiplying swizzles for NEON

src/opts/SkSwizzler_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 SkSwizzler_opts_DEFINED
+#define SkSwizzler_opts_DEFINED
+
+#include "SkColorPriv.h"
+
+namespace SK_OPTS_NS {
+
+// These variable names in these functions just pretend the input is BGRA.
+// They work fine with both RGBA and BGRA.
+
+#if defined(SK_ARM_HAS_NEON)
+
+// Rounded divide by 255, (x + 127) / 255
+static uint8x8_t div255_round(uint16x8_t x) {
+    // result = (x + 127) / 255
+    // result = (x + 127) / 256 + error1
+    //
+    // error1 = (x + 127) / (255 * 256)
+    // error1 = (x + 127) / (256 * 256) + error2
+    //
+    // error2 = (x + 127) / (255 * 256 * 256)
+    //
+    // The maximum value of error2 is too small to matter.  Thus:
+    // result = (x + 127) / 256 + (x + 127) / (256 * 256)
+    // result = ((x + 127) / 256 + x + 127) / 256
+    // result = ((x + 127) >> 8 + x + 127) >> 8
+    //
+    // Use >>> to represent "rounded right shift" which, conveniently,
+    // NEON supports in one instruction.
+    // result = ((x >>> 8) + x) >>> 8
+    //
+    // Note that the second right shift is actually performed as an
+    // "add, round, and narrow back to 8-bits" instruction.
+    return vraddhn_u16(x, vrshrq_n_u16(x, 8));
+}
+
+// Scale a byte by another, (x * y + 127) / 255
+static uint8x8_t scale(uint8x8_t x, uint8x8_t y) {
+    return div255_round(vmull_u8(x, y));
+}
+
+template <bool kSwapRB>
+static void premul_xxxa_should_swaprb(uint32_t dst[], const uint32_t src[], int count) {
+    while (count >= 8) {
+        // Load 8 pixels.
+        uint8x8x4_t bgra = vld4_u8((const uint8_t*) src);
+
+        uint8x8_t b = bgra.val[0],

[mtklein, 2016/01/13 21:04:17]

It'd be nice to harmonize the order we list things now that we've got the names
straight.

Up here we write things in b,g,r,a (0->3) order, but in the portable code we
write things in a,r,g,b (3->0) order.  Either works for me, but as you might
guess, I like a,r,g,b (3->0).

[msarett, 2016/01/13 21:30:27]

Done.

+                  g = bgra.val[1],
+                  r = bgra.val[2],
+                  a = bgra.val[3];
+
+        // Premultiply.
+        b = scale(b, a);
+        g = scale(g, a);
+        r = scale(r, a);
+
+        // Store 8 premultiplied pixels.
+        if (kSwapRB) {
+            bgra.val[0] = r;
+            bgra.val[1] = g;
+            bgra.val[2] = b;
+        } else {
+            bgra.val[0] = b;
+            bgra.val[1] = g;
+            bgra.val[2] = r;
+        }
+        vst4_u8((uint8_t*) dst, bgra);
+        src += 8;
+        dst += 8;
+        count -= 8;
+    }
+
+    while (count --> 0) {

[mtklein, 2016/01/13 21:04:17]

This loop can't be correct.  It's not looking at kSwapRB.   (We can't call
SkPremultiplyARGBInline... that's always returning SkPMColor order.)

Why don't we pull the portable code up top as premul_xxxa_portable, etc, then
from in here you can do your NEON loop, then just

  // Call portable code to finish up the tail of [0,8) pixels.
  auto proc = kSwapRB ? premul_swaprb_xxxa_portable : premul_xxxa_portable;
  proc(dst, src, count);

[msarett, 2016/01/13 21:30:27]

Done.

+        const uint8_t* src8 = (const uint8_t*) src;
+        *dst++ = SkPremultiplyARGBInline(src8[3], src8[0], src8[1], src8[2]);
+        src++;
+    }
+}
+
+static void premul_xxxa(uint32_t dst[], const uint32_t src[], int count) {
+    premul_xxxa_should_swaprb<false>(dst, src, count);
+}
+
+static void premul_swaprb_xxxa(uint32_t dst[], const uint32_t src[], int count) {
+    premul_xxxa_should_swaprb<true>(dst, src, count);
+}
+
+#else
+
+static void premul_xxxa(uint32_t dst[], const uint32_t src[], int count) {
+    for (int i = 0; i < count; i++) {
+        uint8_t a = src[i] >> 24,
+                r = src[i] >> 16,
+                g = src[i] >>  8,
+                b = src[i] >>  0;
+        r = (r*a+127)/255;
+        g = (g*a+127)/255;
+        b = (b*a+127)/255;
+        dst[i] = (uint32_t)a << 24
+               | (uint32_t)r << 16
+               | (uint32_t)g <<  8
+               | (uint32_t)b <<  0;
+    }
+}
+
+static void premul_swaprb_xxxa(uint32_t dst[], const uint32_t src[], int count) {
+    for (int i = 0; i < count; i++) {
+        uint8_t a = src[i] >> 24,
+                r = src[i] >> 16,
+                g = src[i] >>  8,
+                b = src[i] >>  0;
+        r = (r*a+127)/255;
+        g = (g*a+127)/255;
+        b = (b*a+127)/255;
+        dst[i] = (uint32_t)a << 24
+               | (uint32_t)b << 16
+               | (uint32_t)g <<  8
+               | (uint32_t)r <<  0;
+    }
+ }
+
+#endif
+
+static void swaprb_xxxa(uint32_t dst[], const uint32_t src[], int count) {
+    for (int i = 0; i < count; i++) {
+        uint8_t a = src[i] >> 24,
+                r = src[i] >> 16,
+                g = src[i] >>  8,
+                b = src[i] >>  0;
+        dst[i] = (uint32_t)a << 24
+               | (uint32_t)b << 16
+               | (uint32_t)g <<  8
+               | (uint32_t)r <<  0;
+    }
+}
+
+}
+
+#endif // SkSwizzler_opts_DEFINED