[Bug c++/110845] New: Function call when it should inline?

[Bug c++/110845] New: Function call when it should inline?

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110845

            Bug ID: 110845
           Summary: Function call when it should inline?
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: deco33000 at yandex dot com
  Target Milestone: ---

Hi,

The below code is inlined in clang (trunk) but not in gcc (13.2).

The code:

#include <format>
#include <iostream>

using namespace std;

constexpr auto f(bool a) -> void {

    if (a) {

        cout << format("hello 1\n");

    } else {

        cout << format("hello 2\n");
    }

}

template <bool T> 
constexpr auto g() -> void {

    if constexpr (T) {

        cout << format("hello 3\n");

    } else {

        cout << format("hello 4\n");
    }
}


int main() {
    f(true);
    f(false);
    g<true>();
    g<false>();

}

flags = -std=c++23 -O3

You can see/test the Godbolt here:
https://godbolt.org/z/aK86nKzYs

Maybe it is related to "format" because without it, it inlines properly.

[Bug c++/110845] Function call when it should inline?

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110845

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Gcc has an heuristic for main where gcc knows that main is called only once and
does not inline as much into a function that will ever be called exactly once.

My bet if you Rename main to foo, gcc will inline f and g into that.

[Bug c++/110845] Function call when it should inline?

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110845

--- Comment #2 from KL <deco33000 at yandex dot com> ---
Changed main to foo:
same behavior

[Bug c++/110845] Function call when it should inline?

[xry111 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110845

Xi Ruoyao <xry111 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |xry111 at gcc dot gnu.org
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Xi Ruoyao <xry111 at gcc dot gnu.org> ---
(In reply to KL from comment #2)
> Changed main to foo:
> same behavior

It's because you don't have a return statement in foo, causing an undefined
behavior.  And GCC considers undefined behaviors highly improbable to be
executed, so the same logic (not to inline too much into "cold" code paths)
still applies.

If you add the return statement they are inlined:

https://godbolt.org/z/Ko9r4fn3d

[Bug c++/110845] Function call when it should inline?

[deco33000 at yandex dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110845

--- Comment #4 from KL <deco33000 at yandex dot com> ---
My mistake indeed,

You are right everything is OK :+1