[Mesa-dev] Mesa include guard style. (Was: [PATCH] i965/cfg: Remove redundant #pragma once.)

[Francisco Jerez]

Iago Toral <itoral at igalia.com> writes:

> On Wed, 2016-03-09 at 19:04 -0800, Francisco Jerez wrote:
>> Matt Turner <mattst88 at gmail.com> writes:
>> 
>> > On Wed, Mar 9, 2016 at 1:37 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>> >> Iago Toral <itoral at 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.

> Iago
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