Diego Novillo <dnovi...@google.com> a écrit:
> On Mon, Apr 16, 2012 at 10:23, Richard Guenther <rguent...@suse.de> wrote:
[...]
>> I'm not sure PPH will replace PCH - will it?
[...]
> In theory, it may. But it's not in the immediate plans.
>
> The big thing that PCH has going for it is load speed. PCH is almost
> instantaneous, PPH isn't. A 1Gb PCH file can be restored in <1s, the
> corresponding PPH file needs ~8s to restore. The implementation can
> still be tuned for performance, but it will never be as fast as a
> couple of mmap operations.
>
> PPH is a much smaller representation (that 1Gb PCH file can be
> represented in <400Mb in PPH) and more flexible (almost any arbitrary
> subset of headers can be turned into separate PPH images).
>
> So, it's not clear whether we will want to replace PCH with PPH.
> Flexibility costs raw performance. If the performance gap can be
> bridged, we may want to consider it in the future.
I guess it's also worth noting one limitation of PPHs that is, if I
believe the wiki:
In essence, the only headers that can be pre-parsed are those that
produce the same result when they are compiled in isolation or as
part of another translation unit. So, header files that are affected
by pre-processor symbols defined before inclusion are not going to
be considered (e.g., stddef.h).
[1]: http://gcc.gnu.org/wiki/pph#Scope_and_limitations
How hard would it be to drop that limitation?
--
Dodji
