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* [Bug math/17061] New: [ldbl-128/ldbl-128ibm] coshl inaccurate near 0
@ 2014-06-16 21:22 jsm28 at gcc dot gnu.org
2014-06-17 6:56 ` [Bug math/17061] " fweimer at redhat dot com
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: jsm28 at gcc dot gnu.org @ 2014-06-16 21:22 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=17061
Bug ID: 17061
Summary: [ldbl-128/ldbl-128ibm] coshl inaccurate near 0
Product: glibc
Version: 2.19
Status: NEW
Severity: normal
Priority: P2
Component: math
Assignee: unassigned at sourceware dot org
Reporter: jsm28 at gcc dot gnu.org
The ldbl-128 and ldbl-128ibm implementations of coshl have a threshold for
small arguments for which the result (to nearest) is 1. However, in those
cases they return 1 + expm1(abs(x)) rather than 1, and while the threshold is
small enough for cosh(x) (1 + x^2/2 + ...) to be 1, it's not small enough for 1
+ abs(x) to be 1, resulting in large errors. E.g., for ldbl-128:
Failure: Test: cosh (0x1p-72)
Result:
is: 1.00000000000000000000e+00 0x1.00000000000000000100p+0
should be: 1.00000000000000000000e+00 0x1.00000000000000000000p+0
difference: 2.11758236813575084767e-22 0x1.00000000000000000000p-72
ulp : 1099511627776.0000
max.ulp : 1.0000
and for ldbl-128ibm:
Failure: Test: cosh (0x1p-56)
Result:
is: 1.00000000000000001388e+00 0x1.00000000000001000000p+0
should be: 1.00000000000000000000e+00 0x1.00000000000000000000p+0
difference: 1.38777878078144567553e-17 0x1.00000000000000000000p-56
ulp : 562949953421312.0000
max.ulp : 3.0000
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* [Bug math/17061] [ldbl-128/ldbl-128ibm] coshl inaccurate near 0
2014-06-16 21:22 [Bug math/17061] New: [ldbl-128/ldbl-128ibm] coshl inaccurate near 0 jsm28 at gcc dot gnu.org
@ 2014-06-17 6:56 ` fweimer at redhat dot com
2014-06-23 20:21 ` cvs-commit at gcc dot gnu.org
2014-06-23 20:22 ` jsm28 at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: fweimer at redhat dot com @ 2014-06-17 6:56 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=17061
Florian Weimer <fweimer at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Flags| |security-
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* [Bug math/17061] [ldbl-128/ldbl-128ibm] coshl inaccurate near 0
2014-06-16 21:22 [Bug math/17061] New: [ldbl-128/ldbl-128ibm] coshl inaccurate near 0 jsm28 at gcc dot gnu.org
2014-06-17 6:56 ` [Bug math/17061] " fweimer at redhat dot com
@ 2014-06-23 20:21 ` cvs-commit at gcc dot gnu.org
2014-06-23 20:22 ` jsm28 at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2014-06-23 20:21 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=17061
--- Comment #1 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
This is an automated email from the git hooks/post-receive script. It was
generated because a ref change was pushed to the repository containing
the project "GNU C Library master sources".
The branch, master has been updated
via 4648909d56c1e9063017bcddd3271dffadef7cb5 (commit)
from 46a3d3c7d60db15ea65470800bcba695f55ce6f6 (commit)
Those revisions listed above that are new to this repository have
not appeared on any other notification email; so we list those
revisions in full, below.
- Log -----------------------------------------------------------------
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4648909d56c1e9063017bcddd3271dffadef7cb5
commit 4648909d56c1e9063017bcddd3271dffadef7cb5
Author: Joseph Myers <joseph@codesourcery.com>
Date: Mon Jun 23 20:20:10 2014 +0000
Fix cosh spurious underflows from expm1 (bug 16354), inaccurate results
near 0 (bug 17061).
This patch fixes bug 16354, spurious underflows from cosh when a tiny
argument is passed to expm1 and expm1 correctly underflows although
the final result of cosh should be 1. As noted in that bug, some
cases are latent because of expm1 implementations not raising
underflow (bug 16353), but all the implementations are fixed
similarly. They already contained checks for tiny arguments, but the
checks were too late to avoid underflow from expm1 (although they
would avoid underflow from subsequent squaring of the result of
expm1); they are moved before the expm1 calls.
The thresholds used for considering arguments tiny are not
particularly consistent in how they relate to the precision of the
floating-point format in question. They are, however, all sufficient
to ensure that the round-to-nearest result of cosh is indeed 1 below
the threshold (although sometimes they are smaller than necessary).
But the previous logic did not return 1, but the previously computed 1
+ expm1(abs(x)) value. And the thresholds in the ldbl-128 and
ldbl-128ibm code (0x1p-71L - I suspect 0x3f8b was intended in the code
instead of 0x3fb8 - and (roughly) 0x1p-55L) are not sufficient for
that value to be 1. So by moving the test for tiny arguments, and
consequently returning 1 directly now the expm1 value hasn't been
computed by that point, this patch also fixes bug 17061, the (large
number of ulps) inaccuracy for small arguments in those
implementations. Tests for that bug are duly added.
Tested x86_64 and x86 and ulps updated accordingly. Also tested for
mips64 and powerpc32 to validate the ldbl-128 and ldbl-128ibm changes.
[BZ #16354]
[BZ #17061]
* sysdeps/ieee754/dbl-64/e_cosh.c (__ieee754_cosh): Check for
small arguments before calling __expm1.
* sysdeps/ieee754/flt-32/e_coshf.c (__ieee754_coshf): Check for
small arguments before calling __expm1f.
* sysdeps/ieee754/ldbl-128/e_coshl.c (__ieee754_coshl): Check for
small arguments before calling __expm1l.
* sysdeps/ieee754/ldbl-128ibm/e_coshl.c (__ieee754_coshl):
Likewise.
* sysdeps/ieee754/ldbl-96/e_coshl.c (__ieee754_coshl): Likewise.
* math/auto-libm-test-in: Add more cosh tests. Do not allow
spurious underflow for some cosh tests.
* math/auto-libm-test-out: Regenerated.
* sysdeps/i386/fpu/libm-test-ulps: Update.
-----------------------------------------------------------------------
Summary of changes:
ChangeLog | 16 +
NEWS | 20 +-
math/auto-libm-test-in | 13 +-
math/auto-libm-test-out | 732 +++++++++++++++++++--------------
sysdeps/i386/fpu/libm-test-ulps | 1 +
sysdeps/ieee754/dbl-64/e_cosh.c | 4 +-
sysdeps/ieee754/flt-32/e_coshf.c | 2 +-
sysdeps/ieee754/ldbl-128/e_coshl.c | 4 +-
sysdeps/ieee754/ldbl-128ibm/e_coshl.c | 2 +-
sysdeps/ieee754/ldbl-96/e_coshl.c | 2 +-
10 files changed, 458 insertions(+), 338 deletions(-)
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* [Bug math/17061] [ldbl-128/ldbl-128ibm] coshl inaccurate near 0
2014-06-16 21:22 [Bug math/17061] New: [ldbl-128/ldbl-128ibm] coshl inaccurate near 0 jsm28 at gcc dot gnu.org
2014-06-17 6:56 ` [Bug math/17061] " fweimer at redhat dot com
2014-06-23 20:21 ` cvs-commit at gcc dot gnu.org
@ 2014-06-23 20:22 ` jsm28 at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: jsm28 at gcc dot gnu.org @ 2014-06-23 20:22 UTC (permalink / raw)
To: glibc-bugs
https://sourceware.org/bugzilla/show_bug.cgi?id=17061
Joseph Myers <jsm28 at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |FIXED
--- Comment #2 from Joseph Myers <jsm28 at gcc dot gnu.org> ---
Fixed for 2.20.
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2014-06-16 21:22 [Bug math/17061] New: [ldbl-128/ldbl-128ibm] coshl inaccurate near 0 jsm28 at gcc dot gnu.org
2014-06-17 6:56 ` [Bug math/17061] " fweimer at redhat dot com
2014-06-23 20:21 ` cvs-commit at gcc dot gnu.org
2014-06-23 20:22 ` jsm28 at gcc dot gnu.org
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