Date: Fri, 13 Aug 2021 20:30:00 +0200 From: "Stefan Kanthak" <stefan.kanthak@...go.de> To: "Rich Felker" <dalias@...c.org> Cc: "Szabolcs Nagy" <nsz@...t70.net>, <musl@...ts.openwall.com> Subject: Re: [PATCH #2] Properly simplified nextafter() Rich Felker <dalias@...c.org> wrote: > On Fri, Aug 13, 2021 at 02:04:51PM +0200, Stefan Kanthak wrote: >> Szabolcs Nagy <nsz@...t70.net> wrote on 2021-08-10 at 23:34: >> >> >* Stefan Kanthak <stefan.kanthak@...go.de> [2021-08-10 08:23:46 +0200]: >> >> <https://git.musl-libc.org/cgit/musl/plain/src/math/nextafter.c> >> >> has quite some superfluous statements: >> >> >> >> 1. there's absolutely no need for 2 uint64_t holding |x| and |y|; >> >> 2. IEEE-754 specifies -0.0 == +0.0, so (x == y) is equivalent to >> >> (ax == 0) && (ay == 0): the latter 2 tests can be removed; >> > >> > you replaced 4 int cmps with 4 float cmps (among other things). >> > >> > it's target dependent if float compares are fast or not. >> >> It's also target dependent whether the FP additions and multiplies >> used to raise overflow/underflow are SLOOOWWW: how can you justify >> them, especially for targets using soft-float? > > On a target with fenv, raising the exception flags is mandatory and > something must be done to make that happen. Performing the arithmetic > is far faster than prodding at the status flag registers explicitly. I did NOT propose the latter; I just questioned whether/why you use two multiplications to detect underflow. JFTR: FORCE_EVAL(dummy+0.0); raises both overflow and underflow, without multiplication. > On a target without fenv (all the existing soft float targets), it > should be valid for the compiler just not to emit them, since they > have no side effects. I think we could make FORCE_EVAL expand to > nothing on such targets (if the FE_* macros are not defined). This is > probably worth doing. Correct, that's the right thing to do there. [ buggy compiler ] > It may be possible to reduce the number of such ops too; not sure. But > there's no way to eliminate them. It's definitely possible to do a strength reduction and get rid of the multiplications. Stefan
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