PING**2 On Wed, Jan 17, 2018 at 8:02 PM, Janne Blomqvist <blomqvist.ja...@gmail.com> wrote: > PING > > On Fri, Dec 29, 2017 at 8:28 PM, Janne Blomqvist > <blomqvist.ja...@gmail.com> wrote: >> On Fri, Dec 29, 2017 at 7:17 PM, Thomas Koenig <tkoe...@netcologne.de> wrote: >>> Hi Janne, >>> >>>> Using pointer sized variables (e.g. size_t / ptrdiff_t) when the >>>> variables are used as array indices allows accessing larger arrays, >>>> and can be a slight performance improvement due to no need for sign or >>>> zero extending, or masking. >>> >>> >>> Unless I have missed something, all the examples are for cases where >>> the array is of maximum size GFC_MAX_DIMENSIONS. >> >> Many, but not all. >> >>> This is why they >>> were left as int in the first place (because it is unlikely we will have >>> arrays of more than 2^31-1 dimensions soon :-). >>> >>> Do you really think this change is necessary? If not, I'd rather avoid >>> such a change. >> >> I'm not planning on supporting > 2**31-1 dimensions, no. :) >> >> But even if we know that the maximum value is always going to be >> smaller, by using pointer-sized variables the compiler can generate >> slightly more efficient code. >> >> See e.g. https://godbolt.org/g/oAvw5L ; in the functions with a loop, >> the ones which use pointer-sized indices have shorter preambles as >> well as loop bodies. And for the simple functions that just index an >> array, by using pointer-sized indices there is no need to zero the >> upper half of the registers. >> >> I mean, it's not a huge improvement, but it might be a tiny one in some >> cases. >> >> Also, by moving the induction variable from the beginning of the >> function into the loop header, it makes it easier for both readers and >> the compiler to see the scope of the variable. >> >> -- >> Janne Blomqvist > > > > -- > Janne Blomqvist
-- Janne Blomqvist
