[PATCH] Improve performance of strncpy

[Wilco Dijkstra]

> Ondřej Bílka wrote:
> On Thu, Sep 11, 2014 at 08:37:17PM +0100, Wilco Dijkstra wrote:
> > I did a quick experiment with strcpy as it's simpler. Replacing it
> > with memcpy (d, s, strlen (s) + 1) is 3 times faster even on strings
> > of 16Mbytes! Perhaps more surprisingly, it has similar performance on
> > these huge strings as an optimized strcpy.
> >
> What architecture? This could also happen because memcpy has special
> case to handle large strings that speeds this up. Its something that I
> tried in one-pass strcpy but it harms performance as overhead of checking
> size is bigger than benefit of larger size.

The 3x happens on all 3 ISAs I tried. On ARM the memcpy/strlen variant
even beats the optimized strcmp case for most sizes, on x64 it runs at
about 80% of the optimized strcpy for sizes above 4KB.

> > So the results are pretty clear, if you don't have a super optimized
> > strcpy, then strlen+memcpy is the best way to do it.
> >
> It is not that clear as you spend considerable amount of time on small
> lenghts, what is important is constant overhead of strcpy startup.
> However this needs platform specific tricks to decide which alternative
> is fastest.

The overheads are relatively small on modern cores. The memcpy/strlen
is always faster than the single loop for lengths larger than 8-16.

Wilco