Can C++ compilers optimize repeated virtual function calls on the same pointer? [duplicate]

问题

Suppose I have the following code

void f(PolymorphicType *p)
{
    for (int i = 0; i < 1000; ++i)
    {
        p->virtualMethod(something);
    }
}

Will the compiler's generated code dereference p's vtable entry for virtualMethod 1 or 1000 times? I am using Microsoft's compiler.

edit

here is the generated assembly for the real-world case I'm looking at. line->addPoint() is the virtual method of concern. I have no assembly experience, so I'm going over it slowly...

; 369  :        for (int i = 0; i < numPts; ++i)

    test    ebx, ebx
    je  SHORT $LN1@RDS_SCANNE
    lea edi, DWORD PTR [ecx+32]
    npad    2
$LL3@RDS_SCANNE:

; 370  :        {
; 371  :            double *pts = pPoints[i].SystemXYZ;
; 372  :            line->addPoint(pts[0], pts[1], pts[2]);

    fld QWORD PTR [edi+8]
    mov eax, DWORD PTR [esi]
    mov edx, DWORD PTR [eax+16]
    sub esp, 24                 ; 00000018H
    fstp    QWORD PTR [esp+16]
    mov ecx, esi
    fld QWORD PTR [edi]
    fstp    QWORD PTR [esp+8]
    fld QWORD PTR [edi-8]
    fstp    QWORD PTR [esp]
    call    edx
    add edi, 96                 ; 00000060H
    dec ebx
    jne SHORT $LL3@RDS_SCANNE
$LN314@RDS_SCANNE:

; 365  :        }

回答1:

In general, no, it is not possible. The function could destroy *this and placement-new some other object derived from the same base in that space.

Edit: even easier, the function could just change p. The compiler cannot possibly know who has the address of p, unless it is local to the optimization unit in question.

回答2:

Impossible in general, but there are special cases that can be optimized, especially with inter-procedural analysis. VS2012 with full optimizations and whole-program optimization compiles this program:

#include <iostream>

using namespace std;

namespace {
struct A {
  virtual void foo() { cout << "A::foo\n"; }
};

struct B : public A {
  virtual void foo() { cout << "B::foo\n"; }
};

void test(A& a) {
  for (int i = 0; i < 100; ++i)
    a.foo();
}
}

int main() {
  B b;
  test(b);
}

to:

01251221  mov         esi,64h  
01251226  jmp         main+10h (01251230h)  
01251228  lea         esp,[esp]  
0125122F  nop  
01251230  mov         ecx,dword ptr ds:[1253044h]  
01251236  mov         edx,12531ACh  
0125123B  call        std::operator<<<std::char_traits<char> > (012516B0h)  
01251240  dec         esi  
01251241  jne         main+10h (01251230h)

so it's effectively optimized the loop to:

for(int i = 0; i < 100; ++i)
  cout << "B::foo()\n";

来源：https://stackoverflow.com/questions/14756958/can-c-compilers-optimize-repeated-virtual-function-calls-on-the-same-pointer

标签

c++

performance

polymorphism

vtable