Richard Biener wrote: > On Wed, Sep 14, 2016 at 3:45 PM, Jakub Jelinek <ja...@redhat.com> wrote: > > On Wed, Sep 14, 2016 at 03:41:33PM +0200, Richard Biener wrote: > >> > We've seen several different proposals for where/how to do this > >> > simplification, why did you > >> > say strlenopt is best? It would be an unconditional strchr (a, 0) -> a + > >> > strlen (a) rewrite, > >> > ie. completely unrelated to what strlenopt does. We do all the other > >> > simplifications based > >> > on constant arguments in builtins.c and gimple-fold.c, why is strchr (s, > >> > 0) different? >>> > >> I was thinking about the case where strlen opt already knows strlen > >> (a). But sure, gimple-fold.c > >> works as well. > > > > I think for the middle-end, using strchr (a, 0) as canonical instead of a + > > strlen (a) > > is better, and at expansion time we can decide what to use (a + strlen (a) > > if you'd expand strlen inline, rather than as a function call, or > > __rawmemchr (which if libc is sane should be fastest), or strchr, or a + > > strlen (a)).
__rawmemchr is not the fastest on any target I tried, including x86, so GLIBC's current default behaviour of always using __rawmemchr is inefficient. GCC doesn't support __rawmemchr at all, so the strlen optimization can't optimize it. strchr is significantly slower than strlen, so generating that is a bad idea too. So the only reasonable optimization is to always emit a + strlen (a). > OTOH that then argues for doing it in strlenopt because that knows > whether we maybe > already computed strlen (a) (which might have other uses than adding to a). strlenopt can already change strchr (a, 0) into strlen (a) + a when strlen has been called before it, so that part is already done. However it doesn't optimize a strlen after a strchr, so if the expansion is done late you'd end up with a redundant strlen. That means the expansion would need to happen either before or very early in strlenopt (likely an extra pass at init time to avoid upsetting the strlen optimization - we want to treat any strchr as a real strlen). Wilco
