Do loads and math in parallel in SkColorXform_opts

src/opts/SkColorXform_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 SkColorXform_opts_DEFINED
 #define SkColorXform_opts_DEFINED
 
@@ skipping 173 matching lines @@
 }
 
 template <const float (&linear_from_curve)[256], Sk4f (*linear_to_curve)(const Sk4f&)>
 static void color_xform_RGB1(uint32_t* dst, const uint32_t* src, int len,
                              const float matrix[16]) {
     // Load transformation matrix.
     auto rXgXbX = Sk4f::Load(matrix + 0),
          rYgYbY = Sk4f::Load(matrix + 4),
          rZgZbZ = Sk4f::Load(matrix + 8);
 
+    // Load linear floats.  Do this once outside the loop.
+    Sk4f reds, greens, blues;
+    if (len >= 4) {
+        reds   = Sk4f{linear_from_curve[(src[0] >>  0) & 0xFF],

[mtklein, 2016/06/22 14:46:54]

Let's make this a lambda to share here and in the loop?

auto load_next_4 = [&reds, &greens, &blues, &src, &len] {
   reds = Sk4f{ ... };
   greens = Sk4f{ ... };
   blues = Sk4f{ ... };
   src += 4;
   len -= 4;
};

if (len >= 4) {
   load_next_4();
}

while (len >= 4) {
   ...
   load_next_4();
}

Same deal with the other two steps even, so that we ultimately see the loop as

if (len >= 4) {
   load_next_4();
   while (len >= 4) {
       transform_4();
       load_next_4();
       store_4();
   }
   transform_4();
   store_4();
}

(old loop equivalent would be this)
while (len >= 4) {
  load_next_4();
  transform_4();
  store_4();
}

[msarett, 2016/06/22 18:34:55]

Nice!  I like how this looks.

+                      linear_from_curve[(src[1] >>  0) & 0xFF],
+                      linear_from_curve[(src[2] >>  0) & 0xFF],
+                      linear_from_curve[(src[3] >>  0) & 0xFF]};
+        greens = Sk4f{linear_from_curve[(src[0] >>  8) & 0xFF],
+                      linear_from_curve[(src[1] >>  8) & 0xFF],
+                      linear_from_curve[(src[2] >>  8) & 0xFF],
+                      linear_from_curve[(src[3] >>  8) & 0xFF]};
+        blues  = Sk4f{linear_from_curve[(src[0] >> 16) & 0xFF],
+                      linear_from_curve[(src[1] >> 16) & 0xFF],
+                      linear_from_curve[(src[2] >> 16) & 0xFF],
+                      linear_from_curve[(src[3] >> 16) & 0xFF]};
+
+        src += 4;
+        len -= 4;
+    }
+
     while (len >= 4) {
-        // Convert to linear.  The look-up table has perfect accuracy.
-        auto reds   = Sk4f{linear_from_curve[(src[0] >>  0) & 0xFF],
-                           linear_from_curve[(src[1] >>  0) & 0xFF],
-                           linear_from_curve[(src[2] >>  0) & 0xFF],
-                           linear_from_curve[(src[3] >>  0) & 0xFF]};
-        auto greens = Sk4f{linear_from_curve[(src[0] >>  8) & 0xFF],
-                           linear_from_curve[(src[1] >>  8) & 0xFF],
-                           linear_from_curve[(src[2] >>  8) & 0xFF],
-                           linear_from_curve[(src[3] >>  8) & 0xFF]};
-        auto blues  = Sk4f{linear_from_curve[(src[0] >> 16) & 0xFF],
-                           linear_from_curve[(src[1] >> 16) & 0xFF],
-                           linear_from_curve[(src[2] >> 16) & 0xFF],
-                           linear_from_curve[(src[3] >> 16) & 0xFF]};
-
         // Apply the transformation matrix to dst gamut.
         auto dstReds   = rXgXbX[0]*reds + rYgYbY[0]*greens + rZgZbZ[0]*blues,
              dstGreens = rXgXbX[1]*reds + rYgYbY[1]*greens + rZgZbZ[1]*blues,
              dstBlues  = rXgXbX[2]*reds + rYgYbY[2]*greens + rZgZbZ[2]*blues;
 
+        // Load floats for the next iteration.  This can happen in parallel with
+        // the math intensive linear_to_curve conversion.
+        reds   = Sk4f{linear_from_curve[(src[0] >>  0) & 0xFF],
+                      linear_from_curve[(src[1] >>  0) & 0xFF],
+                      linear_from_curve[(src[2] >>  0) & 0xFF],
+                      linear_from_curve[(src[3] >>  0) & 0xFF]};
+        greens = Sk4f{linear_from_curve[(src[0] >>  8) & 0xFF],
+                      linear_from_curve[(src[1] >>  8) & 0xFF],
+                      linear_from_curve[(src[2] >>  8) & 0xFF],
+                      linear_from_curve[(src[3] >>  8) & 0xFF]};
+        blues  = Sk4f{linear_from_curve[(src[0] >> 16) & 0xFF],
+                      linear_from_curve[(src[1] >> 16) & 0xFF],
+                      linear_from_curve[(src[2] >> 16) & 0xFF],
+                      linear_from_curve[(src[3] >> 16) & 0xFF]};
+
         // Convert to dst gamma.
         dstReds   = linear_to_curve(dstReds);
         dstGreens = linear_to_curve(dstGreens);
         dstBlues  = linear_to_curve(dstBlues);
 
         // Clamp floats to byte range.
         dstReds   = clamp_0_to_255(dstReds);
         dstGreens = clamp_0_to_255(dstGreens);
         dstBlues  = clamp_0_to_255(dstBlues);
 
         // Convert to bytes and store to memory.
         auto rgba = (Sk4i{(int)0xFF000000}          )
                   | (SkNx_cast<int>(dstReds)        )
                   | (SkNx_cast<int>(dstGreens) <<  8)
                   | (SkNx_cast<int>(dstBlues)  << 16);
         rgba.store(dst);
 
         dst += 4;
         src += 4;
         len -= 4;
     }
 
+    // Complete the final set of four pixels.
+    auto dstReds   = rXgXbX[0]*reds + rYgYbY[0]*greens + rZgZbZ[0]*blues,
+         dstGreens = rXgXbX[1]*reds + rYgYbY[1]*greens + rZgZbZ[1]*blues,
+         dstBlues  = rXgXbX[2]*reds + rYgYbY[2]*greens + rZgZbZ[2]*blues;
+
+    dstReds   = linear_to_curve(dstReds);
+    dstGreens = linear_to_curve(dstGreens);
+    dstBlues  = linear_to_curve(dstBlues);
+
+    dstReds   = clamp_0_to_255(dstReds);
+    dstGreens = clamp_0_to_255(dstGreens);
+    dstBlues  = clamp_0_to_255(dstBlues);
+
+    auto rgba = (Sk4i{(int)0xFF000000}          )
+              | (SkNx_cast<int>(dstReds)        )
+              | (SkNx_cast<int>(dstGreens) <<  8)
+              | (SkNx_cast<int>(dstBlues)  << 16);
+    rgba.store(dst);
+    dst += 4;
+
     while (len > 0) {
         // Splat r,g,b across a register each.
         auto r = Sk4f{linear_from_curve[(*src >>  0) & 0xFF]},
              g = Sk4f{linear_from_curve[(*src >>  8) & 0xFF]},
              b = Sk4f{linear_from_curve[(*src >> 16) & 0xFF]};
 
         // Apply transformation matrix to dst gamut.
         auto dstPixel = rXgXbX*r + rYgYbY*g + rZgZbZ*b;
 
         // Convert to dst gamma.
@@ skipping 30 matching lines @@
 }
 
 static void color_xform_RGB1_2dot2_to_srgb(uint32_t* dst, const uint32_t* src, int len,
                                            const float matrix[16]) {
     color_xform_RGB1<linear_from_2dot2, linear_to_srgb>(dst, src, len, matrix);
 }
 
 }  // namespace SK_OPTS_NS
 
 #endif // SkColorXform_opts_DEFINED