[Bug c++/110124] New: Not inlining comparison operator when using std::tie with -std=c++20

[Bug c++/110124] New: Not inlining comparison operator when using std::tie with -std=c++20

[vakili at live dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110124

            Bug ID: 110124
           Summary: Not inlining comparison operator when using std::tie
                    with -std=c++20
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vakili at live dot com
  Target Milestone: ---

Here is a simple class with a operator<():

struct A {
    int a0;
    int a1;
    int a2;

    friend inline constexpr bool operator<(const A& x, const A& y) {
        return std::tie(x.a0, x.a1, x.a2) < std::tie(y.a0, y.a1, y.a2);
    }
};

When compiling this code with -std=c++17 the operator will be inlined at usage
location. But when compiled with -std=c++20 it will not be inlined.

Here is the compiler explorer snippet showing the situation:
https://godbolt.org/z/4KdzWjvx4

The problem exists from g++ 10.0 to the trunk (14.0).

[Bug c++/110124] Not inlining comparison operator when using std::tie with -std=c++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110124

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 55263
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55263&action=edit
original testcase

[Bug c++/110124] Not inlining comparison operator when using std::tie with -std=c++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110124

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The problem is GCC knows that main is only called once so the inlining is done
slightly different there.
If you name the function differently, GCC will do the inlining.
That is:
```
#include <iostream>
#include <tuple>

struct A {
    int a0;
    int a1;
    int a2;

    friend inline constexpr bool operator<(const A& x, const A& y) {
        return std::tie(x.a0, x.a1, x.a2) < std::tie(y.a0, y.a1, y.a2);
    }
};

int h()
{
    A a{15, 30, 45};
    A b{20, 30, 40};

    std::cout << (a < b ? 'T' : 'F');
    return 0;
}
```
You will see it is inlined.

[Bug c++/110124] Not inlining comparison operator when using std::tie with -std=c++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110124

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The reason for the C++20 and C++17 difference is because std::tie creates a
std::tuple and for C++20 has a spaceship operator<=> (instead of operator<) and
that causes needing to another another function after the spaceship operator to
figure out `<`.

The IR for C++20:
              D.70406 = std::operator<=><const int&, const int&, const int&,
const int&, const int&, const int&> (&D.62773, &D.62783);
              D.70405 = std::operator< (D.70406, D.62981);

vs for C++17:
          D.56644 = std::operator< <const int&, const int&, const int&, const
int&, const int&, const int&> (&D.51943, &D.51953);

[Bug c++/110124] Not inlining comparison operator when using std::tie with -std=c++20

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110124

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So basically more code for the C++20 case causes the less inlining into main.
Overall this is not an bug really because GCC treats main as known to be called
only once and anything main calls is not always inlined as much as what would
happen into other functions.