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| author | Fabian Schriever <fabian.schriever@gtd-gmbh.de> | 2024-09-11 16:06:44 +0200 |
|---|---|---|
| committer | Jeff Johnston <jjohnstn@redhat.com> | 2024-09-19 13:58:12 -0400 |
| commit | 5fcf159b990c2192df237c3e08d1f918de60d489 (patch) | |
| tree | 9dafec47084b62c7a3359bd21129e312e4eef360 | |
| parent | 5a9fe580149d302eb64ebec516c173903cd99620 (diff) | |
| download | newlib-5fcf159b990c2192df237c3e08d1f918de60d489.zip newlib-5fcf159b990c2192df237c3e08d1f918de60d489.tar.gz newlib-5fcf159b990c2192df237c3e08d1f918de60d489.tar.bz2 | |
powf: Fix the hi+lo decomposition for 2/(3ln2) (FreeBSD)
The decomposition needs to be into 12+24 bits of precision for extra-
precision multiplication, but was into 13+24 bits. On i386 with -O1 the
bug was hidden by accidental extra precision, but on amd64, in 2^32
trials the bug caused about 200000 errors of more than 1 ulp, with a
maximum error of about 80 ulps. Now the maximum error in 2^32 trials
on amd64 is 0.8573 ulps. It is still 0.8316 ulps on i386 with -O1.
The nearby decomposition of 1/ln2 and the decomposition of 2/(3ln2) in
the double precision version seem to be sub-optimal but not broken.
Reference: https://github.com/freebsd/freebsd-src/commit/b4437c3d322a0f6d23d12b6f76d2fc72d2ff0ec2
Original Author: Bruce Evans
| -rw-r--r-- | newlib/libm/math/ef_pow.c | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/newlib/libm/math/ef_pow.c b/newlib/libm/math/ef_pow.c index 07b225f..4ceba3d 100644 --- a/newlib/libm/math/ef_pow.c +++ b/newlib/libm/math/ef_pow.c @@ -50,8 +50,8 @@ lg2_h = 6.93145752e-01, /* 0x3f317200 */ lg2_l = 1.42860654e-06, /* 0x35bfbe8c */ ovt = 4.2995665694e-08, /* -(128-log2(ovfl+.5ulp)) */ cp = 9.6179670095e-01, /* 0x3f76384f =2/(3ln2) */ -cp_h = 9.6179199219e-01, /* 0x3f763800 =head of cp */ -cp_l = 4.7017383622e-06, /* 0x369dc3a0 =tail of cp_h */ +cp_h = 9.6191406250e-01f, /* 0x3f764000 =12b of cp */ +cp_l = -1.1736857402e-04f, /* 0xb8f623c6 =tail of cp_h */ ivln2 = 1.4426950216e+00, /* 0x3fb8aa3b =1/ln2 */ ivln2_h = 1.4426879883e+00, /* 0x3fb8aa00 =16b 1/ln2*/ ivln2_l = 7.0526075433e-06; /* 0x36eca570 =1/ln2 tail*/ |