int main(int argc,char *argv)
int x = strtol(argv,NULL,10);
printf("result is %d\n",x/16);
The resulting assembly contained no division instructions, nor a call to a divide routine, but there is one instruction that does an arithmetic right shift by 4. Changing x to unsigned changed the instruction to a logical right shift by 4.
It first generates an offset value in ESI: 15 if the value to divide was negative, or 0 if it was positive.
Then it adds this to the original value. This is the cunning part - well I think so anyway. It ensures that for the values where x>>4 would give the wrong value (i.e., -15<=x<0), x is made positive and smaller than 16, so that x>>4 gives the correct value of zero. For all other values, the bottom 4 bits don't affect the result of the division, so it's fine to offset them as well.
(Don't think I've ever seen VC++ generate anything like this, but I'd pretty much always have an explicit shift rather than a divide by 16 so maybe it does and I'd just never noticed. The choice of 16 usually comes from having some 4-bit field somewhere, and when dealing with bitfields you're best off using bitwise instructions.)
Strangely, I can't reproduce the effect with a divide by 16. With GCC 4.5.2 -O2, I see a "shrl $4, %eax" for unsigned, and "sarl $4, %eax" for signed. However, if I divide by 2, the results are different; the function: