got it down to two:
https://play.golang.org/p/jmTqhLGaLY_T
On Fri, Feb 21, 2020 at 11:24 AM Bruno Albuquerque <b...@gmail.com> wrote:
>
> This is interesting. If I simplify the loop to something like this:
>
> nrgbaData := make([]byte, len(rgbData)+(len(rgbData)/3))
>
> _ = nrgbaData[len(rgbData)]
>
> for i := 0; i < len(rgbData); i++ {
> nrgbaData[i] = rgbData[i]
> }
>
> then the bounds check at the line inside the for loop is removed.
>
> But if I change it to this:
>
> nrgbaData := make([]byte, len(rgbData)+(len(rgbData)/3))
>
> _ = nrgbaData[len(rgbData)]
>
> for i := 0; i < len(rgbData); i += 3 {
> nrgbaData[i] = rgbData[i]
> }
>
> then the bounds check is not eliminated.
>
> Considering it is still guaranteed that "i" inside the loop will never be
> equal to or greater than len(rgbData), I do not understand why the bounds
> check is now required.
>
> Any ideas?
>
>
> On Fri, Feb 21, 2020 at 10:07 AM Bruno Albuquerque <b...@gmail.com> wrote:
>>
>> Nope. Bound checks are still there. I am puzzled by this one.
>>
>>
>> On Fri, Feb 21, 2020 at 9:34 AM Sebastien Binet <bi...@cern.ch> wrote:
>>>
>>>
>>>
>>> On Fri, Feb 21, 2020 at 5:36 PM Bruno Albuquerque <b...@gmail.com> wrote:
>>>>
>>>> I wrote some simple code to convert a RGB24 image represented as a []byte
>>>> to a NRGBA image (also as a []byte), like so:
>>>>
>>>> https://play.golang.org/p/Y7ICg7t4_nd
>>>>
>>>> Unfortunately, the performance is lacking here and I am trying to improve
>>>> it. The first low hanging fruit seems to be taking advantage of BCE:
>>>>
>>>> $go test -bench .
>>>> goos: linux
>>>> goarch: amd64
>>>> BenchmarkNRGBA-8 484 2636344 ns/op
>>>>
>>>> $ go test -gcflags="-B" -bench .
>>>> goos: linux
>>>> goarch: amd64
>>>> BenchmarkNRGBA-8 855 1654882 ns/op
>>>>
>>>> Unfortunately, I could not find an incantation that would remove the bound
>>>> checks. My naive attempt was to just do
>>>>
>>>> _ = nrgbaData[len(nrgbaData)-1]
>>>>
>>>> at around line 7 in the program above but this did not help and added one
>>>> extra bound check.
>>>>
>>>> Funny enough, if I do
>>>>
>>>> _ = nrgbaData[len(nrgbaData)]
>>>>
>>>> all the bound checks seem to be eliminated but, obviously, the program
>>>> panics at this line.
>>>
>>> what about:
>>> _ = nrgbaData[:len(nrgbaData)]
>>>
>>> does this also remove the bound checks?
>>>
>>> -s
>
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