https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65143
Bug ID: 65143
Summary: [C++11] missing devirtualization for virtual base in
"final" classes
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: balakrishnan.erode at gmail dot com
When a class is marked final, it can devirtualize access to all base classes as
its layout is known. This is missing in gcc.
struct A
{
int i();
};
struct B : public virtual A
{
int get()
{
return A::i() + 1;
}
};
struct C final : public B
{
int get()
{
return A::i() + 2;
}
};
int foo(C& c)
{
return c.get(); // Need not go via vtable pointer as class C is final
}
int foo(B& b2)
{
return b2.get(); // This has to go via vtable as most derived class can
change the location of A
}
Assembly: Both do virtual dispatch
foo(C&):
subq $8, %rsp
movq (%rdi), %rax
addq -24(%rax), %rdi
call one::A::i()
addq $8, %rsp
addl $2, %eax
ret
foo(B&):
subq $8, %rsp
movq (%rdi), %rax
addq -24(%rax), %rdi
call one::A::i()
addq $8, %rsp
addl $1, %eax
ret
Complete example:
gcc: http://goo.gl/U4KEvj
clang: http://goo.gl/PpQCkd -- Clang does this optimization
