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Re: Exploiting knowing sizes of string.
- From: Jakub Jelinek <jakub at redhat dot com>
- To: OndÅej BÃlka <neleai at seznam dot cz>
- Cc: Joseph Myers <joseph at codesourcery dot com>, gcc at gcc dot gnu dot org, law at redhat dot org
- Date: Thu, 4 Jun 2015 14:59:20 +0200
- Subject: Re: Exploiting knowing sizes of string.
- Authentication-results: sourceware.org; auth=none
- References: <20150604105929 dot GA19141 at domone> <alpine dot DEB dot 2 dot 10 dot 1506041206200 dot 31862 at digraph dot polyomino dot org dot uk> <20150604123319 dot GF10247 at tucnak dot redhat dot com> <20150604125331 dot GA22076 at domone>
- Reply-to: 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.