mpz_gcd_ui(NULL, ...) with small limbs

[Marco Bodrato]

Ciao Marc,

Il 2022-02-08 10:05 Marc Glisse ha scritto:
> Makes sense to me. I am just not sure if 3 is the right number and if
> we are consistent about it. Shouldn't 2 be enough? Or is it to be safe

You are right. 2 should be enough.

I think that if a not realloc-able mpz is used as a target for a 
function, it should contain at least one extra limb. But we are not 
consistent about this.

If ones try
gmp$ grep MPZ_TMP_INIT -r mpz

many +1 are found...

But specifically for the gcd one can find
gmp$ grep MPZ_TMP_INIT -r mpq/aors.c
   MPZ_TMP_INIT (gcd, MIN (op1_den_size, op2_den_size));

> I don't think we need to set SIZ(lw) but it looks cleaner and can't be
> very expensive.

I agree, not really needed, but it's probably better to always pass a 
well defined mpz_t to functions.

>> diff -r ed0406cf3c70 mpz/gcd_ui.c
>> --- a/mpz/gcd_ui.c      Wed Feb 02 19:16:36 2022 +0100
>> +++ b/mpz/gcd_ui.c      Tue Feb 08 09:30:53 2022 +0100
>> @@ -41,7 +41,16 @@
>>   if (v > GMP_NUMB_MAX)
>>     {
>>       mpz_t vz;
>> -      mp_limb_t vlimbs[2];
>> +      mpz_t lw;
>> +      mp_limb_t vlimbs[2], wlimbs[3];
>> +
>> +      if (w == NULL)
>> +       {
>> +         PTR(lw) = wlimbs;
>> +         ALLOC(lw) = 3;
>> +         SIZ(lw) = 0;
>> +         w = lw;
>> +       }
>>       vlimbs[0] = v & GMP_NUMB_MASK;
>>       vlimbs[1] = v >> GMP_NUMB_BITS;
>>       PTR(vz) = vlimbs;

Ĝis,
m