Subject: Re: Reason for cygwin GCC 2.97 non-bootstrap found
From: "Zack Weinberg" <[email-masked]>
To: Richard Henderson <[email-masked]>
Cc: Kelley Cook <[email-masked]>, Cygwin mailing list <[email-masked]>, GCC Bugs <[email-masked]>, GCC Patches <[email-masked]>
Date: Mon, 20 Nov 2000 19:34:49 -0800

On Mon, Nov 20, 2000 at 07:24:14PM -0800, Richard Henderson wrote:
> On Mon, Nov 20, 2000 at 03:42:23PM -0800, Zack Weinberg wrote:
> > +#if use_malloc
> > +  return (char *) malloc (size);
> > +#endif
> [...]
> > +  return res + offset;
> 
> I refuse to believe that there exists any magic size/roundup/offset
> combination that *always* returns a full page of aligned memory.

Laurnyas' version of this patch went through extreme contortions to
figure out where in a N-page chunk of memory an N-1-page chunk of
aligned memory was located, assuming that this would be different each
time.  I thought just the opposite you do, so I took it out - it would
be relatively easy to put it back.

> The only semi-decent solution I can think of is to get N*pagesize+C
> bytes from malloc, which can then be delivered as N' aligned data pages,
> plus a control block of size C to determine when the entire block may
> be freed.  N' will normally be N-1, but could accidentally be N.

In fact I plan to do this after I figure out just where the labor
split between libiberty and ggc-page.c should be.  I wanted to get the
basic patch out there so it could be tested by people without mmap, or
malloc that page-aligns all page-sized memory requests.

> Moreover, I think the ggc-page mmap optimizations are useful enough
> that I do not want to defer everything to an xvalloc or the like.

They should come back as a side effect of doing the N*pagesize+C thing
you suggest.

zw

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