[Bug tree-optimization/109791] -Wstringop-overflow warning with -O3 and _GLIBCXX_USE_CXX11_ABI=0

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109791

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |amacleod at redhat dot com,
                   |                            |rguenth at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-05-10

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  This is a missed optimization, we fail to optimize the loop guard

<bb 16> [local count: 329643239]:
_4 = (unsigned long) &MEM <char[3]> [(void *)&str + 2B];
_6 = (unsigned long) __i_14;
_50 = -_6;
_100 = _4 + 18446744073709551615;
_40 = _100 - _6;
_41 = _40 > 13;
if (_41 != 0)

with __i_14 being

<bb 11> [local count: 452186132]:
# __i_14 = PHI <&MEM <char[3]> [(void *)&str + 1B](10), &MEM <char[3]> [(void
*)&str + 2B](9)>

I'll note that the strlen pass runs before VRP (but after DOM), but I'll
also note that likely ranger isn't very good with these kind of
"symbolic" ranges?  How would we handle this?  Using two
relations, __i_14 >= &str + 1 && __i_14 <= &str + 2?

DOM has

Optimizing block #16

1>>> STMT 1 = &MEM <char[3]> [(void *)&str + 2B] ge_expr __i_14
1>>> STMT 1 = &MEM <char[3]> [(void *)&str + 2B] ne_expr __i_14
1>>> STMT 0 = &MEM <char[3]> [(void *)&str + 2B] eq_expr __i_14
1>>> STMT 1 = &MEM <char[3]> [(void *)&str + 2B] gt_expr __i_14
1>>> STMT 0 = &MEM <char[3]> [(void *)&str + 2B] le_expr __i_14
Optimizing statement _4 = (unsigned long) &MEM <char[3]> [(void *)&str + 2B];
LKUP STMT _4 = nop_expr &MEM <char[3]> [(void *)&str + 2B]
2>>> STMT _4 = nop_expr &MEM <char[3]> [(void *)&str + 2B]
Optimizing statement _6 = (unsigned long) __i_14;
LKUP STMT _6 = nop_expr __i_14
2>>> STMT _6 = nop_expr __i_14
Optimizing statement _50 = -_6;
 Registering value_relation (_6 pe64 __i_14) (bb16) at _6 = (unsigned long)
__i_14;
LKUP STMT _50 = negate_expr _6
2>>> STMT _50 = negate_expr _6
Optimizing statement _100 = _4 + 18446744073709551615;
LKUP STMT _100 = _4 plus_expr 18446744073709551615
2>>> STMT _100 = _4 plus_expr 18446744073709551615
Optimizing statement _40 = _100 - _6;
 Registering value_relation (_100 < _4) (bb16) at _100 = _4 +
18446744073709551615;
LKUP STMT _40 = _100 minus_expr _6
2>>> STMT _40 = _100 minus_expr _6
Optimizing statement _41 = _40 > 13;
LKUP STMT _41 = _40 gt_expr 13
2>>> STMT _41 = _40 gt_expr 13
LKUP STMT _40 le_expr 14
Optimizing statement if (_41 != 0)

Visiting conditional with predicate: if (_41 != 0)

With known ranges
        _41: [irange] bool VARYING