On 13 March 2016 at 11:29, Ian Romanick <i...@freedesktop.org> wrote: > On 03/11/2016 03:46 PM, Eric Anholt wrote: >> Ian Romanick <i...@freedesktop.org> writes: >> >>> On 03/10/2016 05:53 PM, Francisco Jerez wrote: >>>> Iago Toral <ito...@igalia.com> writes: >>>> >>>>> On Wed, 2016-03-09 at 19:04 -0800, Francisco Jerez wrote: >>>>>> Matt Turner <matts...@gmail.com> writes: >>>>>> >>>>>>> On Wed, Mar 9, 2016 at 1:37 PM, Francisco Jerez <curroje...@riseup.net> >>>>>>> wrote: >>>>>>>> Iago Toral <ito...@igalia.com> writes: >>>>>>>> >>>>>>>>> On Tue, 2016-03-08 at 17:42 -0800, Francisco Jerez wrote: >>>>>>>>>> brw_cfg.h already has include guards, remove the "#pragma once" which >>>>>>>>>> is redundant and non-standard. >>>>>>>>> >>>>>>>>> FWIW, I think using both #pragma once and include guards is a way to >>>>>>>>> keep portability while still getting the performance advantage of >>>>>>>>> #pragma once where it is supported. >>>>>>>>> >>>>>>>> It's highly unlikely to make any significant difference on any >>>>>>>> reasonably modern compiler. I cannot measure any change in compilation >>>>>>>> time locally from my cleanup. >>>>>>>> >>>>>>>>> Also it seems that we do the same thing in many other files... >>>>>>>>> >>>>>>>> Really? I'm not aware of any other file where we use both. >>>>>>> >>>>>>> There are quite a few in glsl/ >>>>>> >>>>>> Heh, apparently you're right. Anyway it seems rather pointless to use >>>>>> '#pragma once' in a bunch of scattered header files with the expectation >>>>>> to gain some speed, the improvement from a single header file is so >>>>>> minuscule (if it will make any difference at all on a modern compiler >>>>>> and compilation workload, which I doubt) that we would have to use it >>>>>> universally in order to have the chance to measure any improvement. >>>>>> >>>>>> Can we please just decide for one of the include guard styles and use it >>>>>> consistently? Given that the majority of header files in the Mesa >>>>>> codebase use old-school define guards, that it's the only standard >>>>>> option, that it has well-defined semantics in presence of file copies >>>>>> and hardlinks, and that the performance argument against it is rather >>>>>> dubious (although I definitely find '#pragma once' prettier and more >>>>>> concise), I'd vote for using preprocessor define guards universally. >>>>>> >>>>>> What do other people think? >>>>> >>>>> I think we have to use define guards necessarily since #pragma once is >>>>> not standard even it it has wide support. So the question is whether we >>>>> want to use only define guards or define guards plus #pragma once. I am >>>>> fine with doing only define guards as you propose. >>>> >>>> *Shrug* I have the impression that the only real advantage of '#pragma >>>> once' is that you no longer need to do the ifndef/define dance, so I >>>> don't think I can see much benefit in doing both. >>> >>> Several compilers will cache the file name where '#pragma once' occurs >>> and never read that file again. A #include of a file previously seen >>> with '#pragma once' becomes a no-op. Since the file is never read, the >>> compiler avoids all the I/O and the parsing. That is true of MSVC and, >>> I thought, some versions of GCC. As Iago points out, some compilers >>> ignore the #pragma altogether. Since Mesa supports (or does it?) some >>> of these compilers, we have to have the ifdef/define/endif guards. >> >> Compilers have noticed that ifdef/define/endif is a thing and optimized >> it, anyway. >> >> https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html > > That's cool! I don't think GCC did that when I looked into this in > 2010. It sounds like the #pragma actually breaks the GCC optimization, > so let's get rid of them all.
Just to reignite this, I don't this statement is any way true. using #pragma once doesn't break GCC optimisation, the optimisation isn't useful in the presence of #pragma once, as gcc will never ever read those files again, so there is no need to do it. Dave. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
