[Bug middle-end/91490] New: [9/10 Regression] bogus argument missing terminating nul warning on strlen of a flexible array member

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91490

            Bug ID: 91490
           Summary: [9/10 Regression] bogus argument missing terminating
                    nul warning on strlen of a flexible array member
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The strlen call in f() below compiles with no warning and is successfully
folded to a constant, but the equivalent call in g() triggers two spurious
instances of the same warning and is not folded.

The warning is new in GCC 9 so GCC 8 compiles both functions without one, and
folds neither call,

$ cat a.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout a.c
struct A { char n, s[]; };

const struct A a1 = { 3, "321" };

int f (void)
{
  return __builtin_strlen (a1.s);   // no warning, folded to 3
}

const struct A a2 = { 3, { 3, 2, 1, 0 } };

int g (void)
{
  return __builtin_strlen (a2.s);   // bogus warning, not folded
}

a.c: In function ‘g’:
a.c:14:30: warning: ‘strlen’ argument missing terminating nul
[-Wstringop-overflow=]
   14 |   return __builtin_strlen (a2.s);   // bogus warning, not folded
      |                            ~~^~
a.c:10:16: note: referenced argument declared here
   10 | const struct A a2 = { 3, { 3, 2, 1, 0 } };
      |                ^~
a.c:14:30: warning: ‘strlen’ argument missing terminating nul
[-Wstringop-overflow=]
   14 |   return __builtin_strlen (a2.s);   // bogus warning, not folded
      |                            ~~^~
a.c:10:16: note: referenced argument declared here
   10 | const struct A a2 = { 3, { 3, 2, 1, 0 } };
      |                ^~

;; Function f (f, funcdef_no=0, decl_uid=1910, cgraph_uid=1, symbol_order=1)

f ()
{
  <bb 2> [local count: 1073741824]:
  return 3;

}



;; Function g (g, funcdef_no=1, decl_uid=1914, cgraph_uid=2, symbol_order=3)

g ()
{
  long unsigned int _1;
  int _3;

  <bb 2> [local count: 1073741824]:
  _1 = __builtin_strlen (&a2.s);
  _3 = (int) _1;
  return _3;

}