[Bug tree-optimization/78327] Improve VRP for ranges of signed char

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78327

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|-Walloca-large-than false   |Improve VRP for ranges of
                   |positives due to bad range  |signed char
                   |info for signed integers in |
                   |[-TYPE_MAX + N, N]          |
           Severity|normal                      |enhancement

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The problem is related to anything described here.

The problem is VRP is not adding an assert in the true case of for n_9 usage:
  <bb 2>:
  n.0_1 = (unsigned char) n_9(D);
  _2 = n.0_1 + 125;
  _3 = (signed char) _2;
  if (_3 < 0)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  _15 = (int) n_9(D);
  _17 = _15 + 128;
  _19 = (long unsigned int) _17;

  <bb 4>:
  # prephitmp_20 = PHI <_19(3), 128(2)>

------ CUT -----
Basically when we look at the if statement of if (_3 < 0) we don't recognize it
is doing ((signed char)(((unsigned char) n_9) + 125)) < 0.

This should be an easy thing to implement in VRP.

Here is a simple testcase without warnings.
void link_failure ();
void g (signed char n)
{
  if (n < -125 || 2 < n) n = 0;

  if ((n + 128) > 130)
    link_failure();
}

void h (signed char n)
{
  if (n < -125 || 1 < n) n = 0;

  if ((n + 128) > 129)
    link_failure();
}
void g1 (int n)
{
  if (n < -125 || 2 < n) n = 0;

  if ((n + 128) > 130)
    link_failure();
}

void h1 (int n)
{
  if (n < -125 || 1 < n) n = 0;

  if ((n + 128) > 129)
    link_failure();
}


Again this has nothing to with anti-range as there is no ranges being produced
here by VRP for g.