On 03/11/2016 03:46 PM, Eric Anholt wrote: > Ian Romanick <[email protected]> writes: > >> On 03/10/2016 05:53 PM, Francisco Jerez wrote: >>> Iago Toral <[email protected]> writes: >>> >>>> On Wed, 2016-03-09 at 19:04 -0800, Francisco Jerez wrote: >>>>> Matt Turner <[email protected]> writes: >>>>> >>>>>> On Wed, Mar 9, 2016 at 1:37 PM, Francisco Jerez <[email protected]> >>>>>> wrote: >>>>>>> Iago Toral <[email protected]> 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.
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