On Fri, Jan 20, 2017 at 06:27:19AM +0100, Jason A. Donenfeld wrote: > This is most certainly not the case. Read closely. > > I use a different variable for each function. get_random_u32 only > touches batched_entropy_u32, and get_random_u64 only touches > bached_entropy_u64. Under no circumstances will one function use the > other's batched entropy. > > I use a union so that only one struct must be declared. If you think > that's unclear, I can declare two separate structs instead. Let me > know which strategy you prefer -- what I have now, or splitting it > into two structs.
What I would probably do is just use one array and one array index, denominated in 32-bit words, and just grab two 32-bit words for the 64-bit case. Optimizing away the overhead of assembling two 32-bit words for a 64-bit word, and the possibility that we might have to touch two cache lines instead of one --- is it really worth it? Compare that to the fact that you're wasting up to 66% of the generated words in the batched entropy array, and certainly on average you're wasting CPU cycles, even if you are reducing the cost of calling get_random_u{32,64} by a handful of cycles.... - Ted