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

From: Göran Weinholt
Subject: bug#14916: Fixnum procedures can be made to return non-fixnums
Date: Sat, 17 Aug 2013 09:55:14 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.4 (gnu/linux)

Mark H Weaver <address@hidden> writes:

> Göran Weinholt <address@hidden> 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.


Göran Weinholt <address@hidden>
"I knew it! He's stepping up his game! We must match
his game with... MUCH MORE GAME!" -- The Monarch

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