Re: crypto_scrypt-sse.c speedup

[Colin Percival <cperciva@tarsnap.com>]

On 11/15/12 20:50, Solar Designer wrote:
> What's the point in having blockmix_salsa8() use externally-provided
> temporary space (the X variable)?  Is it to ensure X is properly
> aligned (regardless of stack (mis)alignment)?  Is it to ensure there's
> no cache tag conflict between X and other arrays, in case we're on a
> machine with low L1 data cache associativity?

I'm pretty sure there was no point. :-)

To be honest, I didn't spend a huge amount of time optimizing this code...

> I think having X as a local variable lets the compiler fully keep it in
> registers, whereas having it passed into the function by reference may
> result in unnecessary writes into the provided X array before the
> function returns; it may also encourage the compiler to do such writes
> inside the loop, especially since its iteration count is determined by r
> and thus is not known at compile time (might be low).

Makes sense.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid