image: remove bounds check in "At" and "Set" methods, with re slice

[pierrre]

1. What version of Go are you using (go version)?
   1.6
2. What operating system and processor architecture are you using (go env)?
   linux amd64

I just saw this https://go-review.googlesource.com/#/c/20654/

Could we rewrite

func (p *RGBA64) RGBA64At(x, y int) color.RGBA64 {
    if !(Point{x, y}.In(p.Rect)) {
        return color.RGBA64{}
    }
    i := p.PixOffset(x, y)
    return color.RGBA64{
        uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1]),
        uint16(p.Pix[i+2])<<8 | uint16(p.Pix[i+3]),
        uint16(p.Pix[i+4])<<8 | uint16(p.Pix[i+5]),
        uint16(p.Pix[i+6])<<8 | uint16(p.Pix[i+7]),
    }
}

to

func (p *RGBA64) RGBA64At(x, y int) color.RGBA64 {
    if !(Point{x, y}.In(p.Rect)) {
        return color.RGBA64{}
    }
    i := p.PixOffset(x, y)
    s := p.Pix[i:i+8]
    return color.RGBA64{
        uint16(s[0])<<8 | uint16(s[1]),
        uint16(s[2])<<8 | uint16(s[3]),
        uint16(s[4])<<8 | uint16(s[5]),
        uint16(s[6])<<8 | uint16(s[7]),
    }
}

I did some benchmarks on my package https://github.com/pierrre/imageserver/tree/master/image/internal (that borrows code from image).
And I see interesting results.

[ianlancetaylor]

CC @nigeltao

[nigeltao]

Yeah, I'd be willing to accept a CL, if the results are sufficiently interesting.

[pierrre]

How to check if bounds check are really removed?

Should I read assembly code with go build -gcflags="-S"?
What do they look like?

[randall77]

On Mon, Mar 21, 2016 at 1:15 AM, Pierre Durand notifications@github.com
wrote:

How to check if bounds check are really removed?

Should I read assembly code with go build -gcflags="-S"?
What do they look like?

Yes, that will work. They are just a compare and branch where the
destination is a call to panicindex (or panicslice, for slice bounds
checks).

We could use a -d=bounds to print all the places where a bounds check
happens. We have -d=nil and -d=wb for nil checks and write barriers,
respectively. It might be kind of tricky to implement though.

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#14884 (comment)

[brtzsnr]

FWIW, the correct line to use is s := p.Pix[i:i+8:len(p.Pix)].

[gopherbot]

Change https://golang.org/cl/136796 mentions this issue: image: add benchmarks for At and Set methods

[gopherbot]

Change https://golang.org/cl/137495 mentions this issue: image: optimize bounds checking for At and Set methods