It's a well-known fact in numerics that you can just test all f32s (https://randomascii.wordpress.com/2014/01/27/theres-only-four-billion-floatsso-test-them-all/) but you can't test all f64s -- for the same reason, no you won't practically run out of u64s (and, indeed, u64 is sufficient to address any memory that can ever be constructed according to known physics).
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