Assembly of constructor for virtual inheritance - c++

Question

Here's a simple inheritance usinig a virtual base class (code available on Compiler Explorer).
class B {
public:
int i = 1;
};
class D : virtual public B {
public:
int j = 2;
};
void Assign(B *b) {
b->i = 2;
}
int main() {
B *b = new D();
Assign(b);
return 0;
}
The assembly listing of the main() function looks like this:
09 main: # #main
10 push rbp
11 mov rbp, rsp
12 sub rsp, 32
13 mov eax, 16
14 mov edi, eax
15 mov dword ptr [rbp - 4], 0
16 call operator new(unsigned long)
17 xor esi, esi
18 mov ecx, 16
19 mov edx, ecx
20 mov rdi, rax
21 mov qword ptr [rbp - 24], rax # 8-byte Spill
22 call memset
23 mov rdi, qword ptr [rbp - 24] # 8-byte Reload
24 call D::D() [complete object constructor]
25 xor ecx, ecx
26 mov eax, ecx
27 mov rdx, qword ptr [rbp - 24] # 8-byte Reload
28 cmp rdx, 0
29 mov qword ptr [rbp - 32], rax # 8-byte Spill
30 je .LBB1_2
31 mov rax, qword ptr [rbp - 24] # 8-byte Reload
32 mov rcx, qword ptr [rax]
33 mov rcx, qword ptr [rcx - 24]
34 add rax, rcx
35 mov qword ptr [rbp - 32], rax # 8-byte Spill
36 .LBB1_2:
37 mov rax, qword ptr [rbp - 32] # 8-byte Reload
38 mov qword ptr [rbp - 16], rax
39 mov rdi, qword ptr [rbp - 16]
40 call Assign(B*)
41 xor eax, eax
42 add rsp, 32
43 pop rbp
44 ret
What is the effect of line 27-38 of the assembly?
What is the value of rax in line 29?
Why is there a branch statement?

Answer 1

The effect of lines 27-38 is to convert a D * to a B *. Because B is a virtual base class, it can have a variable offset from the start of D. Those 12 lines calculate where the B object is, in an unoptimized way.
The value of eax on line 29 is 0 (see lines 25-26).
The branch statement on line 30 is the result of a NULL pointer check. If the pointer to D is NULL, the conversion to a B * will also be NULL and the extra code to determine the correct offset is not wanted in that case.