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Tricky float -> byte conversion in store_8888.

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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>
This commit is contained in:
Mike Klein 2016-12-02 14:22:57 -05:00 committed by Skia Commit-Bot
parent c2881e9b40
commit 3e05671ace
1 changed files with 11 additions and 4 deletions
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15
src/opts/SkRasterPipeline_opts.h
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@ -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) {