Re: [PATCH v4 04/12] crypto: ecc - Implement vli_mmod_fast_521 for NIST p521

Lukas Wunner
2024-03-03T11:05:19Z

On Thu, Feb 29, 2024 at 09:19:59PM -0500, Stefan Berger wrote:
> +static void vli_mmod_fast_521(u64 *result, const u64 *product,
> + const u64 *curve_prime, u64 *tmp)
> +{
> + const unsigned int ndigits = 9;
> + size_t i;
> +
> + for (i = 0; i < ndigits; i++)
> + tmp[i] = product[i];
> + tmp[8] &= 0x1ff;

Hm, the other vli_mmod_fast_*() functions manually unroll those loops.
Wondering if that would make sense here as well? It's also possible
to tell gcc to unroll a loop with a per-function...

__attribute__((optimize("unroll-loops")))

...but I'm not sure about clang portability.

> @@ -941,6 +966,12 @@ static bool vli_mmod_fast(u64 *result, u64 *product,
> + case 9:
> + if (!strcmp(curve->name, "nist_521")) {
> + vli_mmod_fast_521(result, product, curve_prime, tmp);
> + break;
> + }
> + fallthrough;

If you reorder patch 4 and 5, you could check for curve->nbits == 521 here,
which might be cheaper than the string comparison.

> -#define ECC_MAX_DIGITS (512 / 64) /* due to ecrdsa */
> +#define ECC_MAX_DIGITS (576 / 64) /* due to NIST P521 */

Maybe DIV_ROUND_UP(521, 64) for clarity?

Thanks,

Lukas