Probably there is bug in VC 64-bit compiler (in all versions)

Compile and run the following code with speed optimization (-O2):

#include <stdio.h>

typedef int (*fp)(const unsigned char *buf, unsigned int pos, unsigned int
num);

int f(const unsigned char *buf, unsigned int pos, unsigned int num)
{
 int sum = 0;
 for (; num != 0; num--)
   sum += buf[(size_t)pos++];  
 return sum;
}

fp t = f;

int main()
{
 unsigned char buffer[1] = { 0 };
 unsigned int pos = 0x80000000;
 return t(buffer - pos, pos, 1);
}

Bug description:
pos is unsigned int, but VC compiler uses  
movsxd    r9, edx
command to extend from unsigned int to size_t