RE: [PATCH] Use strlen when searching for a nul char

["Wilco Dijkstra" <wdijkstr at arm dot com>]

> Joseph Myers wrote:
> On Wed, 7 Oct 2015, pinskia@gmail.com wrote:
> 
> > > On Oct 7, 2015, at 7:30 AM, Wilco Dijkstra <wdijkstr@arm.com> wrote:
> > >
> > > Expand strchr (s, '\0') in C/C++ to use strlen. This is faster on most
> > > targets as strlen is a simpler function. Passes GLIBC tests. I'm
> > > planning to do the same for strrchr, strchrnul and rawmemchr in future
> > > patches as people frequently use all of these to find the end of a
> > > string.
> > >
> > > OK for commit?
> >
> > Shouldn't this also be an optimization inside gcc if not already?
> 
> I agree; I'd rather discourage the addition of such optimizations as
> inlines in glibc unless they depend on information that's inherently not
> available to GCC (e.g. properties of architecture-specific function
> implementations in glibc).  As I said before in
> <https://sourceware.org/ml/libc-alpha/2015-06/msg00146.html> the aim
> should be to make the GNU system as a whole as good as possible.

Does this mean we no longer care about supporting older GCC versions
efficiently in GLIBC?

Ie. can we remove bits/string2.h and just do a trivial implementation of
the inlines in string/string-inlines.c?

> (And please accompany performance claims such as "faster on most targets"
> with figures from the benchtests or a reason it's hard to produce such
> figures.  In this case I think the transformation could be justified for
> GCC as something architecture-independent without specific performance
> claims - strlen being inherently simpler because it only ever has to
> search for 0 bytes, so it should never be asymptotically slower than the
> alternative and may be faster.)

I did benchmark it of course, however the existing benchmarks don't address
constant arguments at all. Strlen is a good 2x faster than strchr on Cortex-A57
for larger sizes (for really small sizes strchr is sometimes better but that
just shows that strlen could be further improved for small sizes).

Wilco