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Re: [PATCH] round-ash, a rounding arithmetic shift operator
From: |
Andy Wingo |
Subject: |
Re: [PATCH] round-ash, a rounding arithmetic shift operator |
Date: |
Tue, 22 Feb 2011 20:16:42 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux) |
On Tue 22 Feb 2011 18:54, Mark H Weaver <address@hidden> writes:
> Andy Wingo <address@hidden> writes:
>
>> On Tue 15 Feb 2011 10:49, Mark H Weaver <address@hidden> writes:
>>> The first patch is trivial, but there for the sake of correctness.
>>
>> Please apply, thanks.
>
> Ludo applied this before the 2.0.0 release.
Ah, cool.
> (define (round-ash n count)
> (let ((r (ash n count)))
> (if (and (negative? count)
> (bit-set? (- -1 count) n)
> (or (odd? r) (< (first-set-bit n) (- -1 count))))
> (1+ r)
> r)))
Thanks for all the explanation.
You might get good results with (logand n (ash 1 X)), given that both
logand and ash have opcodes. Dunno.
> Having said all this, let's hold off on this patch for now. I'm
> modifying my bigfloat module to do strictly correct rounding, and that
> may require something slightly more powerful than ash and round-ash. In
> particular, I it may be necessary to have variants that return the
> remainder as well as the quotient, or maybe just some partial
> information about the remainder.
OK, will hold off then. Happy hacking!
Andy
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