Re: issue BOGUS-FIXNUMS (initial draft)

[Rob.MacLachlan@WB1.CS.CMU.EDU]

Striking the FIXNUM type specifier doesn't mean fixnums cease to exist: it
just means this implementation detail is somewhat better hidden.  There
would be nothing to prevent an application that currently uses FIXNUM from
doing:
    (deftype fixnum () '(signed-byte 16))

Or whatever property the program was assuming that FIXNUM had.

Any compiler that can't recognize this deftype as a subtype of its internal
fixnum type (if it in fact is) is broken (not to say that there aren't lots
of broken compilers).


But I think that the issue isn't quite as clear-cut as the anti-fixnums are
making it.  In many implementations, a fixnum type constraint results in a
>10x performance improvement.  People tuning programs for these
implementations cannot ignore this reality, and need some kind of handle on
a "good" integer subrange.

Even if the FIXNUM type specified were flushed, the constants delimiting
the implementation fixnum range should remain.  Of course, with these
constants, once can always:

    (deftype fixnum () `(integer ,most-negative-fixnum ,most-positive-fixnum))

And of course, people will do this.  And their programs will still run
fast, and will still run with no problem on the vast majority of
implementations with a reasonable fixnum size.
 
  Rob