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
