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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/63989] tree-ssa-strlen.c doesn't handle constant pointer plus and array refs if constant offset is smaller than known constant string length
Date: Thu, 20 Nov 2014 11:20:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63989

--- Comment #2 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 20 Nov 2014, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63989
> 
> Jakub Jelinek <jakub at gcc dot gnu.org> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>              Status|UNCONFIRMED                 |NEW
>    Last reconfirmed|                            |2014-11-20
>      Ever confirmed|0                           |1
> 
> --- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> So, we are talking here about e.g.
> 
> int
> f1 (char *p)
> {
>   __builtin_strcpy (p, "foobar");
>   return __builtin_strlen (p + 2);
> }
> 
> int
> f2 (char *p)
> {
>   __builtin_strcpy (p, "foobar");
>   p[2] = '\0';
>   return __builtin_strlen (p);
> }
> 
> int
> f3 (char *p)
> {
>   __builtin_strcpy (p, "foobar");
>   p[2] = '\0';
>   return __builtin_strlen (p + 3);
> }
> 
> int
> f4 (char *p)
> {
>   __builtin_strcpy (p, "foobar");
>   p[2] = 'z';
>   return __builtin_strlen (p);
> }
> 
> and then also determining using get_string_length (if constant) whether a
> character pointer dereference is known to be zero or known to be non-zero.

Well, we are mainly talking about the case where I had to XFAIL
gcc.dg/strlenopt-8.c for us now inline-expanding a two-character
memcpy to a short store.