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 : }
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.
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";
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