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Date: Tue, 10 Aug 2021 08:23:46 +0200
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: <musl@...ts.openwall.com>
Subject: [PATCH] Properly simplified nextafter()
<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;
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++;
Download attachment "nextafter.patch" of type "application/octet-stream" (540 bytes)
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