[Bug tree-optimization/115034] New: Missed optimization: reduntant store of identical value in the slot

[Bug tree-optimization/115034] New: Missed optimization: reduntant store of identical value in the slot

[xxs_chy at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115034

            Bug ID: 115034
           Summary: Missed optimization: reduntant store of identical
                    value in the slot
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xxs_chy at outlook dot com
  Target Milestone: ---

Godbolt link: https://godbolt.org/z/fdxKaxGoj

```
int src(int* outl, bool c1, bool c2) {
    int a;
    *outl = 0;
    if (c1)
        if (c2) {
            dummy();
            return 0;
        } else {
            a = 1;
        }
    else {
        // we don't need to assign a = 0
        a = 0;
    }
    *outl = a;
    return 0;
}
```

can be transformed into:

```
int tgt(int* outl, bool c1, bool c2) {
    int a;
    *outl = 0;
    if (c1) {
        if (c2) {
            dummy();
            return 0;
        } else {
            a = 1;
        }
    } else {
        return 0;
    }
    *outl = 1;
    return 0;
}
```

Because "*outl = 0" is known at the entry, the path "a = 0 -> *outl = 0" can be
cut off. That is, we can move "*outl = 1" into the path where c1 is true.

[Bug tree-optimization/115034] Missed optimization: reduntant store of identical value in the slot

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115034

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note there is some memory model requirements here that I always forget if this
can happen or not.

[Bug tree-optimization/115034] Missed optimization: reduntant store of identical value in the slot

[xxs_chy at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115034

--- Comment #2 from XChy <xxs_chy at outlook dot com> ---
(In reply to Andrew Pinski from comment #1)
> Note there is some memory model requirements here that I always forget if
> this can happen or not.

Hmm. Could you please provide some documents about the memory model of GCC or
specific constraints about C language? The semantics of IR in the LLVM issue
look good to me, since the store is non-volatile and non-atomic. But I'm not
sure how it would be after lifting to C.

[Bug tree-optimization/115034] Missed optimization: reduntant store of identical value in the slot

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115034

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2024-05-13
     Ever confirmed|0                           |1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
This would work if we'd duplicate the path from a = 0 to *outl = 1; return 0;
but we don't early enough (on GIMPLE) and we don't do that on RTL until
BB reorder where ther's no later pass doing this optimization either.

On GIMPLE we see

  <bb 2> [local count: 1073741824]:
  *outl_4(D) = 0;
  if (c1_6(D) != 0)
    goto <bb 3>; [50.00%]
  else
    goto <bb 5>; [50.00%]
...
  <bb 5> [local count: 965079152]:
  # a_1 = PHI <1(3), 0(2)>
  *outl_4(D) = a_1;
  return 0;

but I think tracer doesn't consider paths to exit aka tail duplication,
likely because on GIMPLE we force a single return block.  There's also
no partial redundant store elimination.