[Bug tree-optimization/110519] New: Optimize loop that only assigns to a local variable

[Bug tree-optimization/110519] New: Optimize loop that only assigns to a local variable

[roland.illig at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110519

            Bug ID: 110519
           Summary: Optimize loop that only assigns to a local variable
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: roland.illig at gmx dot de
  Target Milestone: ---

~~~c
struct symbol {
    struct symbol *next;
};

void f(const struct symbol *sym)
{
    for (const struct symbol *s = sym; s != (void *)0; s = s->next)
        do { } while (0);
}
~~~

The above code was extracted from
<https://github.com/NetBSD/src/blob/f74666e8d086952b27d76155223a5d497e69f5a8/usr.bin/xlint/lint1/decl.c#L1319>.

Several other compilers know that this loop has no side effects, even at low
optimization levels, see <https://godbolt.org/z/Y85EGcW3d>.

[Bug tree-optimization/110519] Optimize loop that only assigns to a local variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110519

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is already done when compiling with c++ frontend ...

For C, it can only done for for loops. Which makes it hard to do with the way
gcc handles loops at this point.

[Bug c/110519] Optimize loop that only assigns to a local variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110519

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |c
           Severity|normal                      |enhancement

[Bug c/110519] Optimize loop that only assigns to a local variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110519

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
See bug 94392 comment #3 on why the general case does not apply to C and why we
decided not enable it when using the C front-end.

[Bug c/110519] Optimize for loop that only assigns to a local variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110519

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Optimize loop that only     |Optimize for loop that only
                   |assigns to a local variable |assigns to a local variable
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-07-02

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. As mentioned the general rule does not apply for C, only applies to
`for` loops.

That is it would be invalid to remove the following loop in C:
```
struct symbol {
    struct symbol *next;
};

void f(const struct symbol *sym)
{
  const struct symbol *s = sym;
  {
start_loop:
     if (s != (void *)0)
     {
        do { } while (0);
        s = s->next;
        goto start_loop;
     }
  }
}
```