https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89924
--- Comment #4 from Jan Hubicka <hubicka at ucw dot cz> ---
And to answer the question about why GCC produces more code, it is
actually speculative devirtualization of the call. GCC determines
the most likely target and inlines it.
foo_virtual(Aint*):
# this tests whether the dynamic type uses
# Aint::operator+=
mov rax, QWORD PTR [rdi]
mov rax, QWORD PTR [rax]
cmp rax, OFFSET FLAT:Aint::operator+=(A const&)
jne .L19
# if so, this is inlined copy of operator +=
push rbx
xor ecx, ecx
mov edx, OFFSET FLAT:typeinfo for Aint
mov esi, OFFSET FLAT:typeinfo for A
mov rbx, rdi
call __dynamic_cast
test rax, rax
je .L20
mov eax, DWORD PTR [rax+8]
add DWORD PTR [rbx+8], eax
pop rbx
ret
# this is the fallback code in case devirtualizaiton
# failed.
.L19:
mov rsi, rdi
jmp rax
-fno-devirtualize-speculatively will lead to same code as Clang does.
Honza
