Re: Making --with-wide-int the default

[David Kastrup <dak@gnu.org>]

Paul Eggert <eggert@cs.ucla.edu> writes:

> Eli Zaretskii wrote:
>> To make it the default for all 32-bit builds, we should make sure it
>> works well on the other supported platforms.  Perhaps Paul could tell
>> which platforms he found this to work on, and then we could decide.
>
> --with-wide-int is not the default because Stefan was worried about
> its performance implications on older, slower 32-bit machines (such as
> Stefan's and/or RMS's laptops at the time, if I recall
> correctly). There shouldn't be any correctness problem with it; it's a
> performance issue.
>
> I typically use 64-bit platforms nowadays, where the issue is
> moot. When I do use 32-bit platforms, I normally configure
> --with-wide-int. There is roughly a 30% CPU hit and maybe a 60% hit on
> virtual memory, but it's worth it to me (I normally don't notice the
> difference).

Ugh.  That would be quite noticeable to me.  While I build my own Emacs,
having to drag around a bundle of homebrewn options just to get sensible
behavior does not appeal to me.

Instead of going to 64-bit unilaterally it would seem to make more sense
to me to degrade gracefully into gmp.  GUILE does that, I think that
XEmacs or SXEmacs can do it, and it's usual for Lisp implementations.
That leaves the memory cost for the 0.01% of uses (if at all) that need
it.  Of course, with a CPU cost hit (which in the common case of
addition/subtraction at natural width would amount to checking the
overflow bit afterwards).

Would also make for a nice speedup of calc (which implements everything
the hard way via small ints and lists, the Emacs 18 way: David Gillespie
dropped off the Earth before I could manage to have him dig up his Emacs
19 modifications using floating point when available).  If one can
disentangle the internals appropriately.  But that's probably easier for
using gmp transparently than for using FP as first approximation.

-- 
David Kastrup