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
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