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Re: Serious bug inn GUILE rational handling
From: |
Han-Wen Nienhuys |
Subject: |
Re: Serious bug inn GUILE rational handling |
Date: |
Sun, 24 Dec 2006 12:45:45 +0100 |
User-agent: |
Thunderbird 1.5.0.8 (X11/20061107) |
Bill Schottstaedt escreveu:
>> I have removed support for the reduced bit, and put the reduction in
>> make_fraction.
>
> I think it was intended that equal? would use scm_i_fraction_equalp
> which reduces both arguments before checking equality. So the
> simplest fix would be to mask off the reduced bit in the cell type
> in the check for cell type equality in scm_equalp. I would hesitate
> to remove support for this bit because it will mean you get gcd
> on every integer divide! The current system already slows Guile
I think thes best quick fix would be to put the bit into the 4th
double cell word.
> down by about 10%. On the race condition, my vage recollection
> is that the "is this safe?" question was mine, and I hoped at that
> time that someone who knew about such things would check it
> out -- I believe (it's been a long time since I looked at this stuff)
> that if that line is not safe, there are a lot more like it scattered
> around Guile, so it's scarcely reason to jettison the entire thing.
In that case, we need to fix the more-like-it stuff. Can you point out
some of those cases?
I'm hesitant to put stuff like this in because
1. it _is_ a race condition
2. if it is triggered, it will be next to impossible to reproduce. In
effect this would lead to a once in a million inexplicable corruption.
And that's really bad.
The correct solution would be a to use a lock-free instruction to
swap the old and reduced forms, but I'm not sure if we have those in
GUILE.
As I explained earlier, the logic for doing reduces or not needs to be
more refined. Any program that does serious arithmetic in fractions
will likely have exploding memory requirements.
--
Han-Wen Nienhuys - address@hidden - http://www.xs4all.nl/~hanwen