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

[Eric Anholt]

Ian Romanick <idr at freedesktop.org> writes:

> On 03/10/2016 05:53 PM, Francisco Jerez wrote:
>> 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.
>
> 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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