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[Bug math/16337] ldbl-128 lgammal overflows for small negative arguments
- From: "cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sourceware dot org
- Date: Sun, 22 Dec 2013 20:50:54 +0000
- Subject: [Bug math/16337] ldbl-128 lgammal overflows for small negative arguments
- Auto-submitted: auto-generated
- References: <bug-16337-131 at http dot sourceware dot org/bugzilla/>
http://sourceware.org/bugzilla/show_bug.cgi?id=16337
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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4f40e4b30754c678b7f93cfb9a545deb534f7e4e
commit 4f40e4b30754c678b7f93cfb9a545deb534f7e4e
Author: Joseph Myers <joseph@codesourcery.com>
Date: Sun Dec 22 20:50:16 2013 +0000
Fix ldbl-128 lgammal for small negative arguments (bug 16337).
This patch fixes bug 16337, ldbl-128 lgammal spurious overflows for
small negative arguments (the arguments in question are already in the
testsuite). The implementation uses the reflection formula to compute
lgamma of negative x from lgamma of -x, effectively resulting in a
calculation -log(x^2) + log(-x); cancellation isn't problematic in
this case (bugs for problematic cancellation in lgamma are 2542, 2543,
2558), but the x^2 calculation can underflow (in which case there is
spurious logic to return an overflowing value - lgamma can only ever
correctly overflow for large positive arguments, though tgamma can
overflow for small arguments of either sign as well as large positive
arguments). The fix is simply to calculate the result directly with
logl when the argument is a small enough negative number.
Tested mips64.
* sysdeps/ieee754/ldbl-128/e_lgammal_r.c (__ieee754_lgammal_r):
Calculate results for small negative arguments directly rather
than using reflection formula with special underflow handling.
-----------------------------------------------------------------------
Summary of changes:
ChangeLog | 5 +++++
NEWS | 2 +-
sysdeps/ieee754/ldbl-128/e_lgammal_r.c | 4 ++--
3 files changed, 8 insertions(+), 3 deletions(-)
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