On Thu, 26 Apr 2012, Diego Novillo wrote: > On 4/26/12 9:35 AM, Diego Novillo wrote: > > On Thu, Apr 26, 2012 at 06:43, Dodji Seketeli<do...@seketeli.org> wrote: > > > > > 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? > > > > It's an explicit non-goal, actually. If you relax this requirements, > > you might as well re-parse the header file. The work needed to make > > flexible PPH images will rob you of most/all the performance you were > > looking for. > > Expanding a bit on this point. > > One of the goals of PPH is to act as a bridge towards C++ modules, which is > currently being discussed for inclusion in the next C++ standard. > > While it is uncertain what modules will look like, we want to use this > experience to inform the committee on the different tradeoffs and issues we > found. We will publishing something shortly, and discussing it at the Prague > meeting. > > I a future world with modules (if it comes to be), files acting as modules > will need to have exactly one meaning, regardless of where they are imported > from or what was imported before them (think Java/Go/Python modules).
And btw - the PCH implementation shares this limitation. All tokens consumed are part of the PCH - changing the state when trying to use the PCH will invalidate it (or crash gcc). In this way PCH is even more limited than PPH because you can only have a single PCH file and that single PCH file must represent the very first tokens of the compilation unit it is used in. Richard. -- Richard Guenther <rguent...@suse.de> SUSE / SUSE Labs SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746 GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer
