Generating N contiguous bits

So say you want to generate a bitmap that is N contiguous bits, without using some kind
of crazy inefficient for loop. Here's a way that I figured out how to do that (not to say I invented it or anything, but I couldn't find it when I searched for it).
Obviously this has compiler and machine dependencies, and probably doesn't work on the Ides of March, so use at your own risk.

In this example, I want to generate 6 bits of 1's (so 0b111111):

#include <stdio.h>
main(){
unsigned short a;
short b;
int n = 6;
b = 1 << n;
a = (unsigned short) -b; // extend the left bits
printf("a is %hx\n", a);
printf("~a is %hx\n", ~a);
a = ~a; // now b should be 0b111111
}


That's probably like 4 or 5 operations, so for 6 bits it probably sucks. But if you use 64 bit long longs and want to generate 47 one's then it will save some cycles.

Isn't playing with bits fun?