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Re: Exploiting knowing sizes of string.


On Thu, Jun 04, 2015 at 02:59:20PM +0200, Jakub Jelinek wrote:
> On Thu, Jun 04, 2015 at 02:53:31PM +0200, OndÅej BÃlka 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;
}


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