[Bug tree-optimization/89134] A missing optimization opportunity for a simple branch in loop

[innat_xue at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89134

--- Comment #8 from Feng Xue <innat_xue at hotmail dot com> ---
My mistake, transformation should be:

void f (std::map<int, int> m)
{
    for (auto it = m.begin (); it != m.end (); ++it) {
        if (b) {
            b = do_something();
        } else {
            ++it;
            break;
        }
    }

    for (; it != m.end (); ++it);  /* get an empty loop */
}


(In reply to Feng Xue from comment #7)
> Even loop contains calls with side effects, but only in one condition branch
> path, we can still do some kind of optimization.
> 
> Suppose a loop as:
> 
> void f (std::map<int, int> m)
> {
>     for (auto it = m.begin (); it != m.end (); ++it) {
>         if (b) {
>             b = do_something();    /* Has effect on b */
>         } else {
>                                    /* No effect on b */
>         }
>     }
> }
> 
> We can transform it to:
> 
> void f (std::map<int, int> m)
> {
>     for (auto it = m.begin (); it != m.end (); ++it) {
>         if (b) {
>             b = do_something();
>             ++it;
>             break;
>         }
>     }
> 
>     for (; it != m.end (); ++it);  /* get an empty loop */
> }
> 
> This code takes less computation, especially when 'b' is always evaluated to
> be false.