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Re: Precompiled headers - still useful feature?
- From: Markus Trippelsdorf <markus at trippelsdorf dot de>
- To: Martin LiÅka <mliska at suse dot cz>
- Cc: GCC Development <gcc at gcc dot gnu dot org>
- Date: Wed, 27 May 2015 11:01:39 +0200
- Subject: Re: Precompiled headers - still useful feature?
- Authentication-results: sourceware.org; auth=none
- References: <55657CCE dot 6090006 at suse dot cz>
On 2015.05.27 at 10:14 +0200, Martin LiÅka wrote:
> I would like to ask folks what is their opinion about support of
> precompiled headers for future releases of GCC. From my point of view,
> the feature brings some speed-up, but question is if it's worth for?
>
> Last time I hit precompiled headers was when I was rewriting memory
> allocation statistics infrastructure, where GGC memory is 'streamed'
> and loaded afterwards in usage of precompiled headers. Because of
> that I was unable to track some pointers that were allocated in the
> first phase of compilation.
>
> There are numbers related to --disable-libstdcxx-pch option:
>
> Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz:
> Boostrap time w/ precompiled headers enabled: 35m47s (100.00%)
> Boostrap time w/ precompiled headers disabled: 36m27s (101.86%)
>
> make -j9 check-target-libstdc++-v3 -k time:
> precompiled headers enabled: 8m11s (100.00%)
> precompiled headers disabled: 8m42s (106.31%)
>
> Intel(R) Core(TM) i5-3320M CPU @ 2.60GHz:
> Boostrap time w/ precompiled headers enabled: 57m35s (100.00%)
> Boostrap time w/ precompiled headers disabled: 57m12s (99.33%)
Measuring the impact on bigger projects that use pch like QT or Boost
would be more informative perhaps.
And until C++ modules are implemented (unfortunately nobody is working
on this AFAIK) pch is still the only option left. So deprecating them
now seem premature.
--
Markus