[Bug tree-optimization/126904] backward threader: no path-sensitive load CSE (value numbering?)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126904

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
          Component|middle-end                  |tree-optimization
                 CC|                            |pinskia at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2026-08-16
     Ever confirmed|0                           |1

--- Comment #1 from Drea Pinski <pinskia at gcc dot gnu.org> ---
Take:
```
void foo (void);

static void __attribute__((noinline))
spin (int n)
{
  while (n)
    ;
}

int t ;
double t1;

void
f (int *p, int k)
{
  t = 1;
  if (*p == k)
    ;
  else
    t = 2;
#ifndef OK
  spin (k);
#endif
  if (*p != k)
    foo ();
}
```

PRE is able to handle it when OK is defined.

I think DOM skips over calls that don't have VDEF while VN only does that for
functions which are marked as const or pure (if I am reading the code
correctly).