On Thu, Jun 04, 2015 at 02:59:20PM +0200, Jakub Jelinek wrote:
> On Thu, Jun 04, 2015 at 02:53:31PM +0200, Ondřej Bílka wrote:
> > On Thu, Jun 04, 2015 at 02:33:19PM +0200, Jakub Jelinek wrote:
> > > On Thu, Jun 04, 2015 at 12:26:03PM +0000, Joseph Myers wrote:
> > > > > Again is this worth a gcc pass?
> > > >
> > > > This isn't a matter of compiler passes; it's additional checks in
> > > > existing
> > > > built-in function handling. Maybe that built-in function handling
> > > > should
> > > > move to the match-and-simplify infrastructure (some, for libm functions
> > > > and bswap, already has) to make this even simpler to implement.
> > >
> > > GCC already has a pass that attempts to track known and earlier computed
> > > lengths of strings, and do various transformations and optimizations based
> > > on that, see the tree-ssa-strlen.c pass. Most of that you really can't do
> > > at the glibc headers level.
> > >
> > Yes, I was writing down ideas that I have and this was one of these. I
> > didn't knew it does transformations, just checked that it doesn't use
> > length from stpcpy or in
>
> It does use length from stpcpy in various cases, but stpcpy needs to be
> prototyped. See gcc/testsuite/gcc.dg/strlenopt* for what it does.
> >
> > int foo(char *s)
> > {
> > int l = strlen (s);
> > char *p = strchr (s,'a');
> > return p+l;
> > }
>
> And what do you want to optimize here? The length from strlen is different
> from the difference between p and s. Furthermore, p can return NULL.
>
> Jakub
Change that into
int foo(char *s)
{
int l = strlen (s);
char *p = memchr (s, 'a', l);
return p+l;
}
