Extended Description

e.g. for this code:

struct A { virtual void a(); };
struct B { virtual void b(); };
struct C : virtual A, virtual B {
C();
virtual void key_function();
virtual void a();
virtual void b();
};

C::C() { a(); b(); }
void C::key_function() {}

the assembly for C::C() at -O3 is

_ZN1CC1Ev: # complete ctor
pushq %rbx
movq %rdi, %rbx
movq $_ZTV1C+40, (%rbx)
movq $_ZTV1C+88, 8(%rbx)
callq _ZN1C1aEv # call to C::a is devirtualized
movq (%rbx), %rax
movq %rbx, %rdi
popq %rbx
jmpq *16(%rax) # call to C::b is not!
...
_ZN1CC2Ev: # base ctor
pushq %rbx
movq %rdi, %rbx
movq (%rsi), %rax
movq %rax, (%rbx)
movq 8(%rsi), %rcx
movq -32(%rax), %rax
movq %rcx, (%rbx,%rax)
movq 16(%rsi), %rax
movq (%rbx), %rcx
movq -40(%rcx), %rcx
movq %rax, (%rbx,%rcx)
movq (%rbx), %rax
callq *(%rax) # looks like even C::a is not devirtualized
movq (%rbx), %rax
movq %rbx, %rdi
popq %rbx
jmpq *16(%rax) # call C::b is not devirtualized

The same pattern holds if I define C::C() as "b(); a();" - only the
first vcall in the complete ctor is devirtualized.