Re: Exploiting knowing sizes of string.

[Jakub Jelinek <jakub at redhat dot com>]

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