[Bug tree-optimization/116835] New: phiprop will prop back into a loop

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116835

            Bug ID: 116835
           Summary: phiprop will prop back into a loop
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
            Blocks: 116821, 116824
  Target Milestone: ---

Take:
```
struct Foo {
  int **p;
  int **q;
};

struct Foo1 {
  int t[4];
};

int 
bar (int b, int c, int d, struct Foo1 *aa)
{
  int *p = &c;
  for (int j = 0; j < 4; ++j)
  {
    p = &aa->t[j];
  }
  return *p;
}
```

phiprop1 does:
```
Inserting PHI for result of load _6 = *p_1;
  for edge defining &c inserting load _4 = c;
  for edge defining p_8 inserting load _3 = *p_8;
_6 = PHI <_4(2), _3(3)>
Removing dead stmt:p_1 = PHI <&c(2), p_8(3)>
```

But this is moving the load into the loop which seems wrong and might cause
other issues.
I am running into a regression due to this when fixing PR 116821 .
PR 116824 will cause this to show up more too.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116821
[Bug 116821] phiprop could be improved to handle PHI<&a, ssa_name>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116824
[Bug 116824] phiprop gets confused with vop phi