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Size = 1 billion
Variant 1 (& power of 2): 15.92
Variant 2 (% size)      : 18.92
Variant 3 (if, subtract): 17.48
Variant 4 (no if)       : 16.76

Size = 4 million
Variant 1 (& power of 2): 2.48
Variant 2 (% size)      : 7.11
Variant 3 (if, subtract): 2.58
Variant 4 (no if)       : 2.52

Size = 1 billion
Variant 1 (& power of 2): 15.92
Variant 2 (% size)      : 18.92
Variant 3 (if, subtract): 17.48
Variant 4 (no if)       : 16.76

Size = 4 million
Variant 1 (& power of 2): 2.48
Variant 2 (% size)      : 7.11
Variant 3 (if, subtract): 2.58
Variant 4 (no if)       : 2.52 

Size = 256K
Variant 1 (& power of 2): 1.24
Variant 2 (% size)      : 6.64
Variant 3 (if, subtract): 1.79
Variant 4 (no if)       : 1.95

Variant 1 (& power of 2): 0.58
Variant 2 (% size)      : 6.56
Variant 3 (if, subtract): 1.25
Variant 4 (no if)       : 1.44

Variant 1 (& power of 2): 15.92
Variant 2 (% size)      : 18.92
Variant 3 (if, subtract): 17.48
Variant 4 (no if)       : 16.76

So it appears that the memory manipulation of the bit array will be the limiting factor and not the hash calculation. So you can choose from the above 4 variants based on:

Variant 1 (& power of 2): 0.58
Variant 2 (% size)      : 6.56
Variant 3 (if, subtract): 1.25
Variant 4 (no if)       : 1.44

Size = 1 billion
Variant 1 (& power of 2): 15.92
Variant 2 (% size)      : 18.92
Variant 3 (if, subtract): 17.48
Variant 4 (no if)       : 16.76

Size = 4 million
Variant 1 (& power of 2): 2.48
Variant 2 (% size)      : 7.11
Variant 3 (if, subtract): 2.58
Variant 4 (no if)       : 2.52

So it appears that at large sizes, the memory manipulation of the bit array will be the limiting factor and not the hash calculation. So you can choose from the above 4 variants based on:

So it appears that the memory manipulation of the bit array will be the limiting factor and not the hash calculation. So unless you really need that little bit of speed improvement, I would suggest the most clear code which is:

    hash = (hash + hash2) % size;

So it appears that the memory manipulation of the bit array will be the limiting factor and not the hash calculation. So you can choose from the above 4 variants based on:

1. Whether you can force the size to be a power of 2 or not.
2. How much you need that extra bit of speed versus how clear/simple you want your code to be.