Re: MPS: bignums

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Mattias Engdegård <mattias.engdegard@gmail.com> writes:

> 7 maj 2024 kl. 11.06 skrev Helmut Eller <eller.helmut@gmail.com>:
>
>> It's not so easy.  A mpz_t is a resizable vector of "limbs".  The memory
>> for the mpz_t struct are managed by MPS but the memory for the limbs
>> isn't.  The function mp_set_memory_functions could perhaps be used to
>> allocate limbs in an MPS managed pool; but these functions include a
>> realloc variant which is not directly supported by MPS.  We would also
>> have to trace the reference to the limbs.
>
> We could use the mpn interface directly but it's less convenient and mpz takes care of boring work, as well as some useful logic so it's probably not worth it.
>
>> The reason why mpz_t in GMP is a mutable type is probably because they
>> need computations of the kind a := a + b a lot when doing crypto stuff
>> and in this case memory can be reused efficiently.
>
> Yes, but our bignums aren't mutable. I think it would probably end up
> cheaper to use scratch mpz for the computation, create a single MPS
> allocation for the bignum, and memcpy the data to it. (Especially
> since bignums are small.)
>
>> I think we need to get finalization not just for bignums but also for
>> hash tables.
>
> Yes, but that's actually necessary: they are mutable and can grow. For bignums, finalisation is accidental.

For the uninitiated: This is not directly related to MPS. Bignums are
the same as before with igc.