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bug#10519: guile and (mini-)gmp

From: Niels Möller
Subject: bug#10519: guile and (mini-)gmp
Date: Tue, 26 Mar 2013 09:17:38 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.2 (usg-unix-v)

Mark H Weaver <address@hidden> writes:

> FYI, I've pushed patches to Guile's git repository (stable-2.0 branch)
> which eliminate the known obstacles to mini-gmp integration.

Out of curiosity, I had a quick look at the patches as posted to the
guile devel list.

For the small integer gcd code, you may want to have a look at the
tricks used in
http://gmplib.org:8000/gmp/file/304af17b9ccc/mpn/generic/gcd_1.c, the
code under GCD_1_METHOD == 2

1. Shift out the least significant bit of both a and b (they're always
   one). Maybe you don't need this of you have some extra bits already
   (signed types, or some bits reserved for type info).

2. Then, the sign bit of a - b correctly gives the result of the
   comparison a < b. Constructing a mask from this difference is then a
   simple right shift (if >> on a signed int is not an arithmetic right
   shift, you need shift and a negation).

3. Use this bit mask when forming the absolute value |a - b|, and when
   swapping values around, to avoid unpredictable branches which
   typically are quite expensive.


Niels Möller. PGP-encrypted email is preferred. Keyid C0B98E26.
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