[Bug tree-optimization/114704] New: Missed optimization : eliminate store if the value is known in all predecessors

[Bug tree-optimization/114704] New: Missed optimization : eliminate store if the value is known in all predecessors

[xxs_chy at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114704

            Bug ID: 114704
           Summary: Missed optimization : eliminate store if the value is
                    known in all predecessors
           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/KEeGTM49E

For code like:
```
void src(int *p){
    if(*p == 0)
        goto then;
    else {
        dummy();
        if(*p == 0)
            goto then;
        else
            return;
    }

then:
    *p = 0; // *p is already 0, it's dead now
}

```
In then basic block, *p is known to be 0 in all predecessors, thus the store
"*p = 0" is redundant.

[Bug tree-optimization/114704] Missed optimization : eliminate store if the value is known in all predecessors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114704

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
           Keywords|                            |missed-optimization

[Bug tree-optimization/114704] Missed optimization : eliminate store if the value is known in all predecessors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114704

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-04-12
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. A more general testcase:
```
void dummy();

void src(int *p, int a){
    int t = *p;
    if(t == a)
        goto then;
    else {
        dummy();
        t = *p;
        if(t == a)
            goto then;
        else
            return;
    }

then:
    *p = t; // *p is already a, it's dead now
}

```

[Bug tree-optimization/114704] Missed optimization : eliminate store if the value is known in all predecessors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114704

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=20999

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Related to PR 20999.

[Bug tree-optimization/114704] Missed optimization : eliminate store if the value is known in all predecessors

[xxs_chy at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114704

--- Comment #3 from XChy <xxs_chy at outlook dot com> ---
(In reply to Andrew Pinski from comment #1)
> Confirmed. A more general testcase:
> ```
> void dummy();
> 
> void src(int *p, int a){
>     int t = *p;
>     if(t == a)
>         goto then;
>     else {
>         dummy();
>         t = *p;
>         if(t == a)
>             goto then;
>         else
>             return;
>     }
> 
> then:
>     *p = t; // *p is already a, it's dead now
> }
> 
> ```

Do you mean "*p = a" at the end?

[Bug tree-optimization/114704] Missed optimization : eliminate store if the value is known in all predecessors

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114704

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to XChy from comment #3)
> Do you mean "*p = a" at the end?

In this case a and t should be the same value :). So it does not matter. I was
showing that sometimes gcc messes up when using the original value.

[Bug tree-optimization/114704] Missed optimization : eliminate store if the value is known in all predecessors

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114704

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
We're not handling "phi translation" in the lookup phase when determining if
there's a redundant store (PHI translation for the virtual operand).  In
particular value-numbering never considers whether an expression
in multiple paths into a CFG merge value-numbers the same.  This is only
done as part of PRE which figures some extra fully redundant expressions.
But the redundant store removal is something done after-the-fact using
just the VN machinery.