[Bug c++/79095] [7 regression] spurious stringop-overflow warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79095

--- Comment #8 from Martin Sebor <msebor at gcc dot gnu.org> ---
(In reply to Jeffrey A. Law from comment #4)
> So the simplified tests are interesting, but ultimately too simplified.

You're right, the test case in comment #2 is oversimplified.  The one in the
thread on gcc-patches should be closer (it includes the ADD_OVERFLOW):

$ cat t.C && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout t.C
typedef __SIZE_TYPE__ size_t;

struct S {
  int *p0, *p1, *p2;

  size_t size () const { return p1 - p0; }

  void f (size_t n) {
    if (n > size ())       // can't happen because
      foo (n - size ());   //   n is in [1, MIN(size() - 1, 3)]
    else if (n < size ())
      bar (p0 + n);
  }

  void foo (size_t n)
  {
    size_t left = (size_t)(p2 - p1);
    if (left >= n)
      __builtin_memset (p2, 0, n * sizeof *p2);
  }

  void bar (int*);
};

void f (S &s)
{
  size_t n = s.size ();
  if (n > 1 && n < 5)
    s.f (n - 1);
}

;; Function void f(S&) (_Z1fR1S, funcdef_no=3, decl_uid=2296, cgraph_uid=3,
symbol_order=3)

Removing basic block 9
Removing basic block 10
Removing basic block 11
void f(S&) (struct S & s)
{
  long unsigned int _1;
  long unsigned int _2;
  int * _7;
  int * _8;
  long int _9;
  long int _10;
  long int _11;
  long int _12;
  long unsigned int _13;
  long unsigned int _17;
  int * _22;
  long int _23;
  long int _24;
  __complex__ long unsigned int _25;
  long unsigned int _28;
  int * _29;

  <bb 2> [100.00%]:
  _7 = MEM[(int * *)s_5(D)];
  _8 = MEM[(int * *)s_5(D) + 8B];
  _9 = (long int) _8;
  _10 = (long int) _7;
  _11 = _9 - _10;
  _12 = _11 /[ex] 4;
  _13 = (long unsigned int) _12;
  _1 = _13 + 18446744073709551614;
  if (_1 <= 2)
    goto <bb 3>; [36.64%]
  else
    goto <bb 8>; [63.36%]

  <bb 3> [36.64%]:
  _25 = ADD_OVERFLOW (_13, 18446744073709551615);
  _2 = REALPART_EXPR <_25>;
  _17 = IMAGPART_EXPR <_25>;
  if (_2 > _13)
    goto <bb 4>; [50.00%]
  else
    goto <bb 6>; [50.00%]

  <bb 4> [18.32%]:
  _22 = MEM[(int * *)s_5(D) + 16B];
  _23 = (long int) _22;
  _24 = _23 - _9;
  if (_24 == -4)
    goto <bb 5>; [33.00%]
  else
    goto <bb 8>; [67.00%]

  <bb 5> [6.05%]:
  __builtin_memset (_22, 0, 18446744073709551612); [tail call]
  goto <bb 8>; [100.00%]

  <bb 6> [18.32%]:
  if (_17 != 0)
    goto <bb 7>; [36.64%]
  else
    goto <bb 8>; [63.36%]

  <bb 7> [6.71%]:
  _28 = _2 * 4;
  _29 = _7 + _28;
  S::bar (s_5(D), _29); [tail call]

  <bb 8> [100.00%]:
  return;

}


In function ‘void S::foo(size_t)’,
    inlined from ‘void S::f(size_t)’ at t.C:10:24,
    inlined from ‘void f(S&)’ at t.C:29:16:
t.C:19:47: warning: ‘void* __builtin_memset(void*, int, long unsigned int)’:
specified size 18446744073709551612 exceeds maximum object size
9223372036854775807 [-Wstringop-overflow=]
       __builtin_memset (p2, 0, n * sizeof *p2);
                                               ^