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Re: Fix zone garbage collector to support PCH
- From: Ian Lance Taylor <ian at wasabisystems dot com>
- To: Geoff Keating <geoffk at geoffk dot org>
- Cc: dberlin at dberlin dot org, gcc-patches at gcc dot gnu dot org
- Date: 04 Mar 2004 07:13:39 -0500
- Subject: Re: Fix zone garbage collector to support PCH
- References: <firstname.lastname@example.org><email@example.com>
Geoff Keating <firstname.lastname@example.org> writes:
> Ian Lance Taylor <email@example.com> writes:
> > I don't know why ggc-page.c bothers to page align the file pointer.
> > The file is just being used for fread() and fwrite() of sizes which
> > appear to normally be less than a page. I can't see why aligning the
> > file pointer provides any benefit, and it obviously makes the
> > resulting PCH file a bit larger. However, I didn't try to change
> > that.
> I believe it's to make mmap work better. It certainly makes it more
> efficient, I'm not sure if there aren't platforms where mmap doesn't
> work at all if you don't do this.
The code in question, in ggc_pch_write_object() and
ggc_pch_total_size() in ggc-page.c, is page aligning in the middle of
the area of the file which is being mapped in. gt_pch_save() ensures
that the file offset starts on a page boundary, which is required by
mmap(). I'm not aware of any mmap() implementation which requires
that the total length be a multiple of the page size, but if we want
to be cautious that too could, and probably should, be handled by
Right now ggc-page.c is page aligning between each set of pages
allocated for a particular size. Thinking about it, I see that some
alignment is required here, to ensure that the next set of objects
starts at an appropriate alignment. But I don't see why page
alignment is required between sets of pages.