[Bug c++/113821] New: Missed optimization for final classes: expensive check for result of dynamic cast

[Bug c++/113821] New: Missed optimization for final classes: expensive check for result of dynamic cast

[janschultke at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113821

            Bug ID: 113821
           Summary: Missed optimization for final classes: expensive check
                    for result of dynamic cast
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janschultke at googlemail dot com
  Target Milestone: ---

Code to reproduce (https://godbolt.org/z/48cabYT78)
===================================================

struct Base {
    virtual ~Base() = default;
};
struct Derived final : Base {};

bool is_derived(Base& a) {
    return dynamic_cast<Derived*>(&a);
}


Expected output (clang)
=======================

is_derived(Base&):
        lea     rax, [rip + vtable for Derived+16]
        cmp     qword ptr [rdi], rax
        sete    al
        ret


Actual output (GCC)
===================

is_derived(Base&):
        sub     rsp, 8
        xor     ecx, ecx
        mov     edx, OFFSET FLAT:typeinfo for Derived
        mov     esi, OFFSET FLAT:typeinfo for Base
        call    __dynamic_cast
        test    rax, rax
        setne   al
        add     rsp, 8
        ret


Explanation
===========

For final classes, checking for success of dynamic_cast is equivalent to
checking whether the vtable pointer equals that of the destination type.

GCC is overly pessimistic by calling __dynamic_cast, which is much more
expensive, I'd imagine. Clang emits the same kind of pessimistic code only when
Derived is not final.

[Bug c++/113821] Missed optimization for final classes: expensive check for result of dynamic cast

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113821

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
           Severity|normal                      |enhancement

[Bug c++/113821] Missed optimization for final classes: expensive check for result of dynamic cast

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113821

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

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 63164.

*** This bug has been marked as a duplicate of bug 63164 ***