Deliberately more obviously 'weird' version, from memory: uint64_t x = ...; for(size_t i = 1; i<UINT_BIT ;i*=2) // loop unroll { unsigned m = UINT_

Deliberately more obviously 'weird' version, from memory: uint64_t x = ...; for(size_t i = 1; i<UINT_BIT ;i*=2) // loop unroll { unsigned m = UINT_MAX / ((1<<i)+1); // constant-fold // divide 2^(2k)-1 by 2^k+1 to get 2^k-1 in each group // 2k-bit group means 2^k-1 sets bottom half x = (x & m) + (x>>i & m); // we can save a AND w/o carry but 1-bit and 2-bit // need 2 ANDs to prevent carry between groups // x = (x + (x>>i)) & m; }