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
