bug#2790: emacs 22.1.1 cannot open 5GB file on 64GB 64-bit GNU/Linux box

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Date: Fri, 27 Mar 2009 22:12:54 -0400
> Cc: 2790@emacsbugs.donarmstrong.com
> 
> > Okay, tried it (emacs-23.0.91), but no luck.  Looks very nice, but
> > finding that large file produced the same error.  The value of
> > 'most-positive-fixnum' prints correctly, though (which is different).
> 
> There was an incorrect check that limited the size to INT_MAX/4
> (i.e. 512MB for systems where ints are 32bit).  I've removed this check
> in the CVS code (see patch below).

The patch below does this:

> -           || st.st_size > INT_MAX / 4)
> +           /* Actually, it should test either INT_MAX or LONG_MAX
> +              depending on which one is used for EMACS_INT.  But in
> +              any case, in practice, this test is redundant with the
> +              one above.
> +              || st.st_size > INT_MAX / 4 */)
>           error ("Maximum buffer size exceeded");

But what about the commentary immediately preceding the modified code:

  The calculations below double the file size twice, so check that it
  can be multiplied by 4 safely.

I'm not sure to which calculations it alludes, but if you think they
are no longer relevant, please remove that part of the comment,
otherwise we will wonder in a couple of years why the code does not do
what the comment says it should.

Personally, I would change INT_MAX/4 to LONG_MAX/4, because that does
TRT on all supported platforms, 32-bit and 64-bit alike (long and int
are both 32-bit wide on 32-bit machines).  That would avoid too
radical changes during a pretest, which is a Good Thing, IMO.

> Note also that when you open large files, it's worthwhile to use
> find-file-literally to be sure it's opened in unibyte mode;
> otherwise it gets decoded which takes ages.

Perhaps the prompt we pop for large file should suggest visiting
literally as an option.

> Also if the file has many lines (my
> 800MB file was made up by copying a C file many times, so it had
> millions of lines), turning off line-number-mode is is needed to recover
> responsiveness when navigating near the end of the buffer.

Perhaps we should make the default value of line-number-display-limit
non-nil, at least in 64-bit builds.