Re:A 100LOC C impl of memset, that is faster than glibc's
by saagarjha on Friday November 12, 2021 @08:51AM
(#29199221)
> on x86, probably "rep movsb" or whatever
Only on recent x86, and with a long list of caveats. Look up discussion about erms online.
> I wonder though: it seems to me that memory bandwidth should far and away be the limiting factor for a memcpy, so I would think even a straight-forward translation of the "trivial" implementation wouldn't be that far off from an "optimal" one. I guess memory prefetching would make a difference, but would minimizing the number of loads/stores (or unrolling the loop) really matter that much?
Memory bandwidth is often the limiting factor, but not always. But your simple byte-by-byte loop is not going to get anywhere near saturating that; you'll need to unroll and use vector instructions, which might dispatch slower but copy several orders of magnitude more data.
Only on recent x86, and with a long list of caveats. Look up discussion about erms online.
> I wonder though: it seems to me that memory bandwidth should far and away be the limiting factor for a memcpy, so I would think even a straight-forward translation of the "trivial" implementation wouldn't be that far off from an "optimal" one. I guess memory prefetching would make a difference, but would minimizing the number of loads/stores (or unrolling the loop) really matter that much?
Memory bandwidth is often the limiting factor, but not always. But your simple byte-by-byte loop is not going to get anywhere near saturating that; you'll need to unroll and use vector instructions, which might dispatch slower but copy several orders of magnitude more data.