nisse at lysator.liu.se
Wed Apr 29 07:34:01 UTC 2015
bodrato at mail.dm.unipi.it writes:
> mpn_neg id defined in gmp.h, where we can not check for HAVE_NATIVE, but I
> suspect that also the C version of mpn_com is faster than mpn_neg (in the
> former, limbs do not depend on one another, no "carry" propagation).
I'd suggest something like this (totally untested)
mpn_neg (mp_ptr rp, mp_srcptr up, mp_size_t n)
/* Low zero limbs are unchanged by negation. */
while (*up == 0)
*rp++ = 0;
/* All zero */
/* First non-zero limb is negated. */
*rp++ = - *up++;
/* Higher limbs get complemented. */
mpn_com (rp, up, n);
In the common case, the bulk of the work is the final mpn_com call. And
if there are many low zeros, the initial loop should make good progress
with reasonably few c/l. (Both mpn_neg and mpn_com require n > 0, right?)
Not sure why we're doing this as an inline function in gmp.h.
Niels Möller. PGP-encrypted email is preferred. Keyid C0B98E26.
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