
Date: Tue, 10 Aug 2021 23:34:55 +0200 From: Szabolcs Nagy <nsz@...t70.net> To: Stefan Kanthak <stefan.kanthak@...go.de> Cc: musl@...ts.openwall.com Subject: Re: [PATCH] Properly simplified nextafter() * Stefan Kanthak <stefan.kanthak@...go.de> [20210810 08:23:46 +0200]: > <https://git.musllibc.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. IEEE754 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. (the i386 machine where i originally tested this preferred int cmp and float cmp was very slow in the subnormal range and iirc it also raises the nonstandard input denormal exception, which is fine i guess. of course soft float abis much prefer int cmp so your code is likely much slower and bigger there). but i'm not against the change, it is likely better on modern machines. did you try to benchmark it? or check the code size? > 3. there's absolutely no need to compare the signs of x and y > with the sign of the direction: its sufficient to test that > direction and sign of x match; > 4. a proper compiler/optimizer should be able to reuse the results > of the comparision (x == y) for (x < y) or (x > y) and > (x == 0.0) for (x < 0.0) or (x > 0.0). > > JFTR: if ((x < 0.0) == (x < y)) is equivalent to > if ((x > 0.0) == (x > y)) > >  /src/math/nextafter.c > +++ +/src/math/nextafter.c > @@ 3,20 +3,15 @@ > double nextafter(double x, double y) > { > union {double f; uint64_t i;} ux={x}, uy={y}; >  uint64_t ax, ay; > int e; > > if (isnan(x)  isnan(y)) > return x + y; >  if (ux.i == uy.i) > + if (x == y) > return y; >  ax = ux.i & 1ULL/2; >  ay = uy.i & 1ULL/2; >  if (ax == 0) { >  if (ay == 0) >  return y; > + if (x == 0.0) > ux.i = (uy.i & 1ULL<<63)  1; >  } else if (ax > ay  ((ux.i ^ uy.i) & 1ULL<<63)) > + else if ((x < 0.0) == (x < y)) > ux.i; > else > ux.i++;
Powered by blists  more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.