[Bug tree-optimization/127103] code from std::vector::push_back can be optimized away

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127103

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2026-08-27
             Status|UNCONFIRMED                 |ASSIGNED
             Blocks|85316                       |
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |pinskia at gcc dot 
gnu.org

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
So these two functions are the same:
```
unsigned t2(unsigned a)
{
    if (a >= __INT_MAX__)
      __builtin_unreachable();
    unsigned t = __INT_MAX__ - a;
    unsigned ttt = a > 1 ? a : 1;
    ttt = ttt < t ? ttt : t;
    unsigned tt = ttt + a;
    return tt;
}

unsigned t(unsigned a)
{
    if (a >= __INT_MAX__)
      __builtin_unreachable();
    unsigned b = a * 2;
    a = ((signed)b) < 0 ? __INT_MAX__ : b;
    a = a < 1 ? 1 : a;
    return a;
}
/*
  b_4 = a_2(D) * 2;
  a_5 = MIN_EXPR <b_4, 2147483647>;
  _8 = MAX_EXPR <a_5, 1>;
*/

```
And then we know that is always `s> 0`.

Note this only works when we know that a < __INT_MAX .

I will take this.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85316
[Bug 85316] [meta-bug] VRP range propagation missed cases