[cfe-dev] Optimizing vcalls from structors and virtual this-adjusting thunks

[Timur Iskhodzhanov]

Hi John,

I've noticed Clang doesn't devirtualize all vcalls in ctors/dtors.

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.

Does this look like a bug to you?
GCC devirtualizes all four calls in this example...

I also have a somewhat related ABI question.
Is there any reason to keep virtual this-adjusting thunks in the
vtable when the class is fully constructed?
I think all the offsets between bases are known statically at the end
of the complete object constructor, so a special "final vtable" with
only static this adjusting thunks can be used instead of a regular
vtable?
Am I missing something?

--
Thanks,
Timur