mpz_gcd_ext(NULL, ...)

Niels Möller nisse at
Fri Nov 25 11:55:18 UTC 2016

Marc Glisse <marc.glisse at> writes:

> a user was asking if we could support calling mpz_gcd_ext with a NULL
> first argument (the gcd), since they are only interested in the
> coefficients s and t and would like to save the unnecessary
> allocation. I doubt it would save that much, but it seems trivial to
> add a check if(g!=NULL) similar to the tests for s and t. Does it make
> sense to you?

Sounds reasonable to me. But then I'd also consider adding a return
value, returning one if the inputs were coprime (gcd == 1), otherwise
zero. Would be useful for mpz_invert. 

If we add a return value, for consistency we should do it for mpz_gcd
too, so one could define mpz_coprime_p as

#define mpz_coprime_p(u,v) mpz_gcd (NULL, (u), (v))

(Adding an integer return value is a breaking ABI change in theory, but
not really in practice, as far as I understand).

It won't save much for the computation, only the final copy. But it's
also a gain to clarity if callers don't have to pass in dummy, unused,
result arguments.


Niels Möller. PGP-encrypted email is preferred. Keyid 368C6677.
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