How to calculate 2^n-1 efficiently without overflow?
- by Ludwig Weinzierl
I want to calculate 2^n-1 for a 64bit integer value.
What I currently do is this
for(i=0; i<n; i++) r|=1<<i;
and I wonder if there is more elegant way to do it.
The line is in an inner loop, so I need it to be fast.
I thought of
r=(1ULL<<n)-1;
but it doesn't work for n=64, because << is only defined
for values of n up to 63.
