Re: 64-bit lossage

[Paul Eggert <eggert@twinsun.com>]

> From: Ken Raeburn <raeburn@raeburn.org>
> Date: Thu, 01 Aug 2002 13:19:44 -0400
> 
> But after looking at it a little, I think doubling the size of a
> Lisp_Object is probably not worth it.

I agree in general, but Emacs on x86 is quite popular, and the current
28-bit address limit is starting to bite.  A few years ago I started
to work on supporting 64-bit Lisp_Object on 32-bit hosts and I got
about half the way through.  It's not that hard, but it is a bit
tedious (among other things, every function really needs to be
prototyped).

Changing the subject slightly, can't we increase address space and
improve performance on both 32- and 64-bit hosts, without widening
Lisp_Object, by moving the 4 tag bits to the low-order end of the
Lisp_Object, and ensuring that all non-Lisp_Int objects are aligned at
a multiple of 16?  That way, we would get all of the 32-bit address
space.  Lisp_Int addition and subtraction would still be fast, since
the Lisp_Int code is zero.  (Multiplication and division would be
slower, but that's rare.)  And we would speed up access to
non-Lisp_Int objects, since the tag bits in many cases could be
removed at zero cost during normal address arithmetic.

> Wider integer support on a 32-bit platform, so larger files can be
> supported, buffer offsets can still be described, etc....

This would require 64-bit ints, yes.  But I'm more worried about the
current 28-bit limit on Lisp object addresses on 32-bit hosts.  That's
only 256 MB.