Most efficient way to set n consecutive bits to 1?

I want to get a function that will set the n last bits of a numerical type to 1. For example:

bitmask (5) = 0b11111 = 31
bitmask (0) = 0

I, first, had this implementation (mask_t is just a typedef around uint64_t):

mask_t bitmask (unsigned short n) {
  return ((((mask_t) 1) << n) - 1;
}

Everything is fine except when the function hit bitmask (64) (the size of mask_t), then I get bitmask (64) = 0 in place of 64 bits set to 1.

So, I have two questions:

1. Why do I have this behavior ? Pushing the 1 by 64 shifts on the left should clear the register and remain with 0, then applying the -1 should fill the register with 1s…

2. What is the proper way to achieve this function ?

You need to sign in to view this answers