[Bug c++/92054] New: `final` does not cause devirtualization of nested calls

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92054

            Bug ID: 92054
           Summary: `final` does not cause devirtualization of nested
                    calls
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Consider the example:


struct A {
    virtual int f() { return 0; }
    virtual int g() { return f() + 40; } 
};

struct B2 final : A {
    int f() override { return 42; }
};

int test(B2& b) {
    return b.g();
}


GCC-10 generates the assembly that does a fair vptr call. However, `B2` is
final, so any call to the virtual functions of `A` end up with a call to the
same function in `B2`. So `B2::g()` should inline the `A::g()` and get
optimized to:

int test(B2& b) { return B2::f() + 40; }


Which is just 82, because `B2::f()` always returns 42.

Godbolt playground: https://godbolt.org/z/PJ4nL-