image/png: use branch-free abs function in abs8

[haruyama480]

This is not a bug issue, but propose of optimization.
So, I change subjects of pr-templates.

overview

In the past CL, a branch-free abs was applied to optimize Paeth filters.
https://codereview.appspot.com/117290043

Applying this to abs8 in write.go also improved the latency of BenchmarkEncodeRGBA by 38%.

result

goos: darwin
goarch: arm64
pkg: image/png
                            │   old.txt   │               new.txt               │
                            │   sec/op    │   sec/op     vs base                │
Paeth-10                      2.536n ± 1%   2.534n ± 0%        ~ (p=0.378 n=10)
DecodeGray-10                 354.2µ ± 1%   354.5µ ± 0%        ~ (p=0.353 n=10)
DecodeNRGBAGradient-10        1.510m ± 0%   1.513m ± 0%   +0.16% (p=0.043 n=10)
DecodeNRGBAOpaque-10          1.212m ± 0%   1.215m ± 0%   +0.28% (p=0.011 n=10)
DecodePaletted-10             190.3µ ± 2%   190.2µ ± 2%        ~ (p=0.684 n=10)
DecodeRGB-10                  1.085m ± 1%   1.085m ± 0%        ~ (p=0.912 n=10)
DecodeInterlacing-10          1.393m ± 0%   1.391m ± 0%        ~ (p=0.052 n=10)
EncodeGray-10                 1.178m ± 0%   1.127m ± 0%   -4.39% (p=0.000 n=10)
EncodeGrayWithBufferPool-10   1.104m ± 0%   1.052m ± 0%   -4.65% (p=0.000 n=10)
EncodeNRGBOpaque-10           3.566m ± 0%   3.449m ± 1%   -3.29% (p=0.000 n=10)
EncodeNRGBA-10                3.891m ± 1%   3.727m ± 0%   -4.23% (p=0.000 n=10)
EncodePaletted-10             1.541m ± 0%   1.535m ± 0%   -0.39% (p=0.011 n=10)
EncodeRGBOpaque-10            3.567m ± 0%   3.445m ± 0%   -3.41% (p=0.000 n=10)
EncodeRGBA-10                 24.58m ± 0%   15.25m ± 1%  -37.97% (p=0.000 n=10)
geomean                       616.8µ        587.4µ        -4.75%

[gopherbot]

Change https://go.dev/cl/534375 mentions this issue: image/png: use branch-free abs function in abs8

[haruyama480]

The fix is a few lines, so I created the CL.

[AlexRouSg]

if you've tested and compared the output of your function to the old one you'll see that your "optimized" function doesn't even do the same thing on values >= 129
https://go.dev/play/p/NCH4u6HM3if

[nakulbajaj]

Given the above, the best optimization I could come up with based on a 2's complement approach was the following. Benchmark result differences weren't statistically significant compared to the branching version, so I think we should leave as-is?

func abs8(d uint8) int {
	m := uint8(int8(d) >> 7)
	return int((d ^ m) - m)
}

[haruyama480]

You are right. My first CL is wrong.

It was embarrassing that I ignored the following code comment because all test passed...

The absolute value of a byte interpreted as a signed int8

[haruyama480]

nakulbajaj's suggestion seems to work.
https://go.dev/play/p/MlsoDr9ZE3N

And,

Benchmark result differences weren't statistically significant compared to the branching version, so I think we should leave as-is?

Yes, I agree that. The differences include even worse cases.

goos: darwin
goarch: arm64
pkg: image/png
                            │  old1.txt   │              new1.txt              │
                            │   sec/op    │   sec/op     vs base               │
Paeth-10                      2.466n ± 1%   2.476n ± 2%       ~ (p=0.288 n=10)
DecodeGray-10                 342.3µ ± 0%   341.9µ ± 1%       ~ (p=0.529 n=10)
DecodeNRGBAGradient-10        1.467m ± 0%   1.463m ± 1%       ~ (p=0.218 n=10)
DecodeNRGBAOpaque-10          1.176m ± 1%   1.177m ± 0%       ~ (p=0.631 n=10)
DecodePaletted-10             183.7µ ± 1%   184.1µ ± 1%       ~ (p=0.353 n=10)
DecodeRGB-10                  1.049m ± 0%   1.064m ± 2%  +1.41% (p=0.000 n=10)
DecodeInterlacing-10          1.346m ± 1%   1.351m ± 1%       ~ (p=0.190 n=10)
EncodeGray-10                 1.138m ± 1%   1.196m ± 1%  +5.07% (p=0.000 n=10)
EncodeGrayWithBufferPool-10   1.072m ± 0%   1.115m ± 1%  +4.05% (p=0.000 n=10)
EncodeNRGBOpaque-10           3.448m ± 1%   3.615m ± 0%  +4.85% (p=0.000 n=10)
EncodeNRGBA-10                3.762m ± 1%   3.997m ± 1%  +6.24% (p=0.000 n=10)
EncodePaletted-10             1.496m ± 1%   1.499m ± 1%       ~ (p=0.143 n=10)
EncodeRGBOpaque-10            3.452m ± 2%   3.619m ± 2%  +4.84% (p=0.000 n=10)
EncodeRGBA-10                 23.89m ± 1%   22.77m ± 1%  -4.72% (p=0.000 n=10)
geomean                       597.4µ        606.8µ       +1.58%

[haruyama480]

Thanks for your confirmation!
If you need, I'm happly to add a test for abs8.
Or, there seems to be no improvement, and I will close CL and this issue.