From: Joseph Myers <joseph@codesourcery.com>
Date: Wed, 9 Dec 2015 21:20:18 +0000 (+0000)
Subject: Fix ldbl-128ibm tanhl inaccuracy for small arguments (bug 19349).
X-Git-Tag: glibc-2.23~140
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ca2fcac629b9b7781ccd52685b28741d89ff128f;p=thirdparty%2Fglibc.git

Fix ldbl-128ibm tanhl inaccuracy for small arguments (bug 19349).

The ldbl-128ibm implementation of tanhl is inaccurate for small
arguments, because it returns x*(1+x) (maybe in an attempt to raise
"inexact") when x itself would be the accurate return value but
multiplying by 1+x introduces large errors.  This patch fixes it to
return x in that case (when the mathematical result is x plus a
negligible remainder on the order of x^3) to avoid those errors.

Tested for powerpc.

	[BZ #19349]
	* sysdeps/ieee754/ldbl-128ibm/s_tanhl.c (__tanhl): Return argument
	when small.
---

diff --git a/ChangeLog b/ChangeLog
index b2eddff236c..b76c54b9a53 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2015-12-09  Joseph Myers  <joseph@codesourcery.com>
 
+	[BZ #19349]
+	* sysdeps/ieee754/ldbl-128ibm/s_tanhl.c (__tanhl): Return argument
+	when small.
+
 	* sysdeps/unix/sysv/linux/i386/kernel-features.h
 	[__LINUX_KERNEL_VERSION >= 0x040300] (__ASSUME_SOCKET_SYSCALL):
 	New macro.
diff --git a/sysdeps/ieee754/ldbl-128ibm/s_tanhl.c b/sysdeps/ieee754/ldbl-128ibm/s_tanhl.c
index 292020cabff..e6457a1c1cf 100644
--- a/sysdeps/ieee754/ldbl-128ibm/s_tanhl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_tanhl.c
@@ -69,7 +69,7 @@ long double __tanhl(long double x)
 	    if (ix<0x3c60000000000000LL) 	/* |x|<2**-57 */
 	      {
 		math_check_force_underflow (x);
-		return x*(one+x);    	/* tanh(small) = small */
+		return x;		/* tanh(small) = small */
 	      }
 	    if (ix>=0x3ff0000000000000LL) {	/* |x|>=1  */
 		t = __expm1l(two*fabsl(x));