Can a compiler inline a virtual function if I use

问题:

I've already read Are inline virtual functions really a non-sense?. But I still have some doubts and found no answer there.

They say that if situation isn't ambiguous, compiler should inline the virtual function.

However:

This can happen only when the compiler has an actual object rather than a pointer or reference to an object.

So what if I have a B class derived from an A one (which contains a virtual void doSth() function) and I use the B* pointer, not the A*:

B* b = new B;

b->doSth();

1. Suppose that the B hasn't any child classes. It's rather obvious (on the compile time) what function should be called. So it's possible to be inlined. Is it in fact?
2. Suppose that the B has some child classes but these classes haven't its own doSth() function. So compiler should "know" that the only function to call is B::doSth(). I guess it doesn't inline though?

回答1:

It doesn't matter whether B has any derived classes. In that situation b points to a B object so the compiler can inline the call.

And surely any decent modern compiler can and will do that in your situation. If you don't use pointers it becomes a whole lot easier. It's not really an "optimization" then. The fact that you can omit a virtual call then becomes obvious by only looking at the AST node at the left side of the .-operator. But if you use pointers, you need to track the dynamic type of the pointee. But modern compilers are capable of that.

EDIT: Some experiment is in order.

// main1.cpp
struct A {
  virtual void f();
};

struct B : A { 
  virtual void f();
};

void g() {
  A *a = new A;
  a->f();

  a = new B;
  a->f();
}

// clang -O2 -S -emit-llvm -o - main1.cpp | c++filt
// ...
define void @g()() {
  %1 = tail call noalias i8* @operator new(unsigned int)(i32 4)
  %2 = bitcast i8* %1 to %struct.A*
  %3 = bitcast i8* %1 to i32 (...)***
  store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* @vtable for A, i32 0, i32 2) to i32 (...)**), i32 (...)*** %3, align 4
  tail call void @A::f()(%struct.A* %2)
  %4 = tail call noalias i8* @operator new(unsigned int)(i32 4)
  %5 = bitcast i8* %4 to i32 (...)***
  store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* @vtable for B, i32 0, i32 2) to i32 (...)**), i32 (...)*** %5, align 4
  %tmp = bitcast i8* %4 to %struct.B*
  tail call void @B::f()(%struct.B* %tmp)
  ret void
}
// ...

As can be seen, clang does direct calls to f, both when a points to a A and when it points to a B. GCC does that too.

回答2:

A virtual member function can be inlined when the vtable is not dereferenced for the call. This can be accomplished by making an explicitly scoped call to the member function.

class A
{
protected:

    int     a;
public:
    inline virtual void Func()
    {
        a = 0;
    }
};

class B : public A
{
public:
    inline virtual void Func()
    {
        a = 1;
    }
};

B   *obj = new B();

obj->Func();    //  Calls B::Func() through vtable;
obj->A::Func(); //  Inlines calls to A::Func();
obj->B::Func(); //  Inlines calls to B::Func();