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>

0 files changed, 0 insertions, 0 deletions