Date: Sun, 15 Aug 2021 12:09:00 -0400 From: Rich Felker <dalias@...c.org> To: Ariadne Conill <ariadne@...eferenced.org> Cc: musl@...ts.openwall.com, Szabolcs Nagy <nsz@...t70.net> Subject: Re: [PATCH #2] Properly simplified nextafter() On Sun, Aug 15, 2021 at 10:52:13AM -0500, Ariadne Conill wrote: > Hi, > > On Sun, 15 Aug 2021, Stefan Kanthak wrote: > > >Szabolcs Nagy <nsz@...t70.net> wrote: > > > >>* Stefan Kanthak <stefan.kanthak@...go.de> [2021-08-15 09:04:55 +0200]: > >>>Szabolcs Nagy <nsz@...t70.net> wrote: > >>>>you should benchmark, but the second best is to look > >>>>at the longest dependency chain in the hot path and > >>>>add up the instruction latencies. > >>> > >>>1 billion calls to nextafter(), with random from, and to either 0 or +INF: > >>>run 1 against glibc, 8.58 ns/call > >>>run 2 against musl original, 3.59 > >>>run 3 against musl patched, 0.52 > >>>run 4 the pure floating-point variant from 0.72 > >>> my initial post in this thread, > >>>run 5 the assembly variant I posted. 0.28 ns/call > >> > >>thanks for the numbers. it's not the best measurment > > > >IF YOU DON'T LIKE IT, PERFORM YOUR OWN MEASUREMENT! > > > >>but shows some interesting effects. > > > >It clearly shows that musl's current implementation SUCKS, at least > >on AMD64. > > I would rather have an implementation that is 3.59 ns/call and is > maintained by somebody who is actually pleasant to talk to. In the > grand scheme of things 3.59 ns/call, and even 8.58 ns/call are not a > big deal for a function like nextafter(). > > If musl does wind up merging this, I intend to revert that merge in > Alpine because I cannot trust the correctness of any code written by > somebody with this attitude. Don't worry, we will evaluate the correctness of anything we do merge and stand by it. I'm open to well-reasoned changes, especially if they improve things more broadly like the isnan comparisons, and have already noted that there's some inconsistency in the existing nextafter/nexttoward functions. But I'm not going to accept any patch that's not making a well-reasoned, isolated, evaluatable, and testable change. And I would very much appreciate dropping exaggerated and hostile claims that something "sucks" or is "brain damaged" in further discussion of the topic. Rich
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