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.
