Paul.Zimmermann at inria.fr
Mon Dec 3 14:06:02 UTC 2018
> Date: Fri, 30 Nov 2018 07:43:40 +0100
> From: Marco Bodrato <bodrato at mail.dm.unipi.it>
> Ciao Paul!
> Il 2018-11-23 13:38 paul zimmermann ha scritto:
> > I presented some work in progress on "Micro-GMP", a modified
> > version of Mini-GMP which works with 16-bit or even 8-bit limbs:
> It is always interesting to see a further development based on some code
> we wrote!
> > https://members.loria.fr/PZimmermann/talks/microgmp.pdf
> In your presentation you write that "27 functions from mini-gmp.c need
> to be modified".
> Of course the code in mini-gmp exploits the fact that mp_limb_t is
> defined as unsigned long, to keep the code as simple and as effective as
> possible. If you need to define a different mp_limb_t type, some
> modifications are obviously needed.
> But... are you sure you need to change that many? I mean:
> - you changed mpz_div_qr_ui, did you really need to change also most of
> the functions mpz_?div_*_ui originally implemented as a simple call to
most changes for the _ui functions (including mpz_div_qr_ui) consist in first
converting the unsigned long argument into a mpz_t using mpz_set_ui, then
calling the corresponding function without _ui.
But indeed I don't remember why I changed the other mpz_?div_*_ui functions.
Maybe because it was simpler to just convert using mpz_set_ui, and do not have
to think at all.
> - for similar reasons, was changing mpz_mul_si really needed? or
it seems mpz_mul_si does not need to be changed. Same for mpz_lcm_ui.
> I looked at the code and I'd suggest a couple of simpler
> mpz_set_si (mpz_t r, signed long int x)
> if (x >= 0)
> mpz_set_ui (r, x);
> else /* (x < 0) */
> mpz_set_ui (r, GMP_NEG_CAST (unsigned long int, x));
> mpz_neg (r, r);
> mpz_cmpabs_ui (const mpz_t u, unsigned long v)
> mpz_t absu;
> mpz_roinit_n (absu, u->_mp_d, GMP_ABS (u->_mp_size));
> return mpz_cmp_ui (absu, v);
indeed this is simpler.
Now there are only 18 that need to be modified.
> You also changed mpn_invert_3by2. Maybe a handful of (mp_limb_t) casts
> could be added to the original function to make it work... but your
> rewrite is simpler and faster. Maybe the same could be done on
> gmp_umul_ppmm? for speed?
indeed, in several places I opted for simplicity, since speed was not
(at first) a goal. But when running "make check" of MPFR with micro-gmp-8,
I now realize speed is also important!
> Unfortunately, writing a micro-gmp implementation outside of the
> project, has the bad effect that you will have to rewrite it for any new
> Your micro- does not benefit of the improvement in mini- since the
> release of GMP-6.1.2, and some of your modifications might be
> superfluous now.
> E.g. you changed in mpz_probab_prime_p the line:
> nm1->_mp_size = mpz_abs_sub_ui (nm1, n, 1);
> mpz_abs (nm1, n);
> mpz_sub_ui (nm1, nm1, 1);
> but the current code reads
> mpz_abs (nm1, n);
> nm1->_mp_d -= 1;
> which works for any limb size (n is odd, -=1 on the lowest limb is
> always possible) and is faster than calling _sub_ui.
> Maybe we can find a strategy to release also micro- as a part of the
> project, reducing code duplication. Would you like to cooperate on this?
yes of course!
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