[Bug tree-optimization/97628] format truncation false positive for O1 and mtune=nocona

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97628

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
             Blocks|                            |85741
      Known to fail|                            |10.2.0, 11.0, 9.3.0
   Last reconfirmed|                            |2020-10-29

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Confirmed with GCC 9 through 11.  The -Wformat-truncation and -Wformat-overflow
warnings rely on the range propagation engine in GCC (EVRP) to determine the
ranges of integer arguments.  The note they print shows the range they obtain
from it.  In this case the range is less than optimal.  GCC 11 adds a new range
engine (the Ranger) capable of computing more accurate ranges on demand.  The
Ranger hasn't been integrated with the strlen/sprintf pass yet but once it is,
I expect this warning to go away.  In the meantime, explicitly asserting just
before the snprintf that the argument is in the expected range avoids the
warning:

          if (num < 10) __builtin_unreachable ();
          snprintf (buf, 10, "%d", num);

as does casting the argument to an unsigned type.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85741
[Bug 85741] [meta-bug] bogus/missing -Wformat-overflow