Ah. Now it makes more sense. I completely abstracted away overflows (again, not a scenario that would happen in practice in any case).
On Sat, Feb 22, 2020 at 3:54 AM Nigel Tao <nigel...@golang.org> wrote: > On 2/22/20, Bruno Albuquerque <b...@gmail.com> wrote: > > https://play.golang.org/p/P2JPI42YJa8 > > > > ... > > > > So, in this simplified case, if the step divides the length evenly, then > > there are no bound checks. If it does not, then bound checks are > inserted. > > > > This seems to be an unnecessary check > > If the step (e.g. 3) does not divide the length evenly, then e.g. "i > += 3" can overflow such that i becomes negative, even though the > "len(a)" in the "i < len(a)" condition is a legitimate array or slice > length: a non-negative int, less than or equal to INT_MAX. > > See: > https://play.golang.org/p/QGNKOtw3m62 > > as the actual relevant thing is to > > make sure that the index is not equal to or greater than the slice > length. > > Either that or I am missing something > > > > On Fri, Feb 21, 2020 at 10:24 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 > >>>> > >>> > > > > -- > > You received this message because you are subscribed to the Google Groups > > "golang-nuts" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to golang-nuts+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > > https://groups.google.com/d/msgid/golang-nuts/CAEd86Tw_1qLzK4HEw9rA06Wqnrtzs6CuzMT6VAK3Y40Bs7yPBw%40mail.gmail.com > . > > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAEd86TxsbiHJH%3DU1LowZP7zPUxETzyw50eNZqWo1nueC6eVQNQ%40mail.gmail.com.
