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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