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From: Richard Stallman <rms@gnu.org>
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Subject: Re: 64-bit lossage
Date: Fri, 2 Aug 2002 16:13:36 -0600 (MDT)
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    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?

A cons cell is 8 bytes, so it would have to be a multiple of 8.

Anyway, is the limiting factor really address space for Lisp objects?
If the issue is the size of a single buffer, that is concerned
with the range of integers, not addresses of Lisp objects.