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author | Mike Klein <mtklein@chromium.org> | 2016-12-02 14:22:57 -0500 |
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committer | Skia Commit-Bot <skia-commit-bot@chromium.org> | 2016-12-02 19:54:39 +0000 |
commit | 3e05671ace65faf4b849275f2eefb534e336e92f (patch) | |
tree | 4eadc1310adc600e344ea3db64e47d6fe83d09d9 /src/opts | |
parent | c2881e9b404d757cb26a1c68ed9c8a51ddd36f6b (diff) |
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>
Diffstat (limited to 'src/opts')
-rw-r--r-- | src/opts/SkRasterPipeline_opts.h | 15 |
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) { |