Tricky float -> byte conversion in store_8888.

In IEEE, for each byte BB, the float 0x470000BB equals 32768.0f + BB*(1/256.0f).

So to turn a [0,1] float into a byte, we can
  - multiply by (255/256.0f) to get into [0,255/256.0f] range,
  - add 32768.0f to get into [32768.0f, 32768.0f + 255/256.0f] range,
  - look at the low byte.
Those first two of course are an FMA.

Using this trick here makes store_8888 measurably faster.  Instead of a FMA then float->int trunc, we do an FMA then a bitwise AND.  Overall the math goes from 4 FMA + 4 trunc + 3 shift to 4 FMA + 3 AND + 3 shift (we can skip the shift for red and the AND for alpha).  As you might guess, AND is cheaper than trunc, so this is a net win.

I should be able to follow up with the same trick in reverse in from_8888().

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Change-Id: I42c8f4a6ea0b6c22160517cf5f9c048f01c9a330
Reviewed-on: https://skia-review.googlesource.com/5540
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

1 files changed, 11 insertions, 4 deletions

diff --git a/src/opts/SkRasterPipeline_opts.h b/src/opts/SkRasterPipeline_opts.h
index 8164d6cac3..14f1255ec8 100644
--- a/src/opts/SkRasterPipeline_opts.h
+++ b/src/opts/SkRasterPipeline_opts.h
@@ -547,11 +547,18 @@ STAGE(load_8888_d) {
     from_8888(load(tail, ptr), &dr, &dg, &db, &da);
 }
 STAGE(store_8888) {
+    auto byte = [](const SkNf& x, int ix) {
+        // Here's a neat trick: 0x47000000 == 32768.0f, and 0x470000ff == 32768.0f + (255/256.0f).
+        auto v = SkNf_fma(255/256.0f, x, 32768.0f);
+        switch (ix) {
+            case 0: return SkNi::Load(&v) & 0xff;  // R
+            case 3: return SkNi::Load(&v) << 24;   // A
+        }
+        return (SkNi::Load(&v) & 0xff) << (8*ix);  // B or G
+    };
+
     auto ptr = *(uint32_t**)ctx + x;
-    store(tail, ( SkNf_round(255.0f, r) << 0
-                | SkNf_round(255.0f, g) << 8
-                | SkNf_round(255.0f, b) << 16
-                | SkNf_round(255.0f, a) << 24 ), (int*)ptr);
+    store(tail, byte(r,0)|byte(g,1)|byte(b,2)|byte(a,3), (int*)ptr);
 }
 
 STAGE(load_tables) {