Tune linear->sRGB constants to round-trip all bytes.

bench/SkRasterPipelineBench.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Benchmark.h"
 #include "SkRasterPipeline.h"
 #include "SkSRGB.h"
@@ skipping 121 matching lines @@
                                   Sk4f dr, Sk4f dg, Sk4f db, Sk4f da) {
     auto A = 1.0f - a;
     r += dr * A;
     g += dg * A;
     b += db * A;
     a += da * A;
 
     st->next(x, r,g,b,a, dr,dg,db,da);
 }
 
-static Sk4f clamp(const Sk4f& x) {
-    return Sk4f::Min(Sk4f::Max(x, 0.0f), 255.0f);
-}
-
 static void SK_VECTORCALL store_srgb(SkRasterPipeline::Stage* st, size_t x,
                                      Sk4f  r, Sk4f  g, Sk4f  b, Sk4f  a,
                                      Sk4f dr, Sk4f dg, Sk4f db, Sk4f da) {
     auto ptr = st->ctx<uint32_t*>() + x;
 
-    r = clamp(sk_linear_to_srgb(r));
-    g = clamp(sk_linear_to_srgb(g));
-    b = clamp(sk_linear_to_srgb(b));
-    a = clamp(         255.0f * a );
-
-    ( SkNx_cast<int>(r)
-    | SkNx_cast<int>(g) << 8
-    | SkNx_cast<int>(b) << 16
-    | SkNx_cast<int>(a) << 24 ).store(ptr);
+    ( sk_linear_to_srgb(r)
+    | sk_linear_to_srgb(g) << 8
+    | sk_linear_to_srgb(b) << 16
+    | Sk4f_round(255.0f*a) << 24).store(ptr);
 }
 
 static void SK_VECTORCALL store_srgb_tail(SkRasterPipeline::Stage* st, size_t x,
                                           Sk4f  r, Sk4f  g, Sk4f  b, Sk4f  a,
                                           Sk4f dr, Sk4f dg, Sk4f db, Sk4f da) {
     auto ptr = st->ctx<uint32_t*>() + x;
 
-    auto rgba = sk_linear_to_srgb({r[0], g[0], b[0], 0});
-    rgba = {rgba[0], rgba[1], rgba[2], 255.0f*a[0]};
-    rgba = clamp(rgba);
+    Sk4i rgba = sk_linear_to_srgb({r[0], g[0], b[0], 0});
+    rgba = {rgba[0], rgba[1], rgba[2], (int)(255.0f * a[0] + 0.5f)};
 
     SkNx_cast<uint8_t>(rgba).store(ptr);
 }
 
 class SkRasterPipelineBench : public Benchmark {
 public:
     SkRasterPipelineBench() {}
 
     bool isSuitableFor(Backend backend) override { return backend == kNonRendering_Backend; }
     const char* onGetName() override { return "SkRasterPipelineBench"; }
 
     void onDraw(int loops, SkCanvas*) override {
         SkRasterPipeline p;
         p.append(load_s_srgb, load_s_srgb_tail,  src);
         p.append(   scale_u8,    scale_u8_tail, mask);
         p.append(load_d_srgb, load_d_srgb_tail,  dst);
         p.append(srcover);
         p.append( store_srgb,  store_srgb_tail,  dst);
 
         while (loops --> 0) {
             p.run(N);
         }
     }
 };
 
 DEF_BENCH( return new SkRasterPipelineBench; )