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Unfortunately, compilers are currently unable to optimize the nextafter
function, even in the degenerate case where the second argument is
explicitly infinite. So, this commit implements this case by hand.
On the following testcase, this gives a 15% speedup.
From Coq Require Import Int63 BinPos PrimFloat.
Definition foo n :=
let eps := sub (next_up one) one in
Pos.iter (fun x => next_down (add x eps)) two n.
Time Eval vm_compute in foo 100000000.
And when looking at the cost of just the allocation-free version of
next_down, the speedup is 1500%. Said otherwise, the latency of next_down
is now on par with the measurement noise due to cache misses and the like.
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