https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89989
Bug ID: 89989
Summary: missed devirtualization opportunity on final function
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Example:
struct Base {
virtual int foo() { return 0; }
int bar() { return foo(); }
};
struct Derived : Base {
int foo() override final { return 1; }
};
int call_foo(Derived& d) { return d.foo(); }
int call_bar(Derived& d) { return d.bar(); }
For call_foo, the devirtualization happens:
call_foo(Derived&):
mov eax, 1
ret
For call_bar, gcc emits a branch to check whether 'd' is in fact a Derived:
call_bar(Derived&):
mov rax, QWORD PTR [rdi]
mov rax, QWORD PTR [rax]
cmp rax, OFFSET FLAT:Derived::foo()
jne .L6
mov eax, 1
ret
.L6:
jmp rax
But Derived::foo() is final, so there can't be any other valid behavior besides
simply returning 1.
Notably, if you add "int bar() { return foo(); }" to Derived as well, then
call_bar also just emits "mov eax, 1".
