bug#14916: Fixnum procedures can be made to return non-fixnums

[Andy Wingo <wingo@pobox.com>]

Hi Mark,

I know you like mathy things and so this might be a project you would
like.  I think the right thing to do here is to redefine fixnum? as

 (= x (logand x #x2fffffff))

on 32-bit targets and 8 more f's for 64-bit targets.  Make sure to get
that inline.  In that way we'll end up unboxing X and doing unboxed
arithmetic on it.  Likewise we can insert a similar check at the end.

Andy

On Sat 17 Aug 2013 09:55, Göran Weinholt <goran@weinholt.se> writes:

> Mark H Weaver <mhw@netris.org> writes:
>
>> Göran Weinholt <goran@weinholt.se> writes:
>>
>>> the fxdiv procedure from (rnrs) fails to check that its result is
>>> representable as a fixnum:
>
>> Hmm.  Currently, our fixnum and flonum operations are implemented in
>> terms of the generic operations, with added checks.  Whereas the most
>> important generic arithmetic operations compile to VM instructions, the
>> fixnum and flonum operations compile into procedure calls to scheme code
>> that performs the checks and then uses the generic ops.
>>
>> Needless to say, this is terribly slow.  I'm reluctant to make that code
>> any slower by adding more checks.
>
> I agree with this sentiment. The fixnum operations are supposed to be
> fast, so making them slower doesn't make sense. There is a delicious
> irony in the fact that the generic operations have all these extra
> checks that would have to be undone by adding more checks afterwards.
>
>> However, in the coming months I intend to reimplement the fixnum and
>> flonum operations, using dedicated instructions in the new RTL VM which
>> will be the basis of Guile 2.2.
>>
>> It would be possible to backport some of this to Guile 2.0 as well, but
>> I'm not sure it's worth the effort.
>>
>> What do you think?
>
> It's better to look toward the future. If Guile 2.2 will be much faster
> then you get more leverage when optimizing the fixnum/flonum operations
> than compared with Guile 2.0.
>
> Regards,