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authorJoseph Myers <joseph@codesourcery.com>2014-03-21 18:13:58 +0000
committerJoseph Myers <joseph@codesourcery.com>2014-03-21 18:13:58 +0000
commitf7be737659813220e1f29c8850c386a9654d549a (patch)
treec047f314e8eca5df155322e4fe9471593fb07b67 /sysdeps/ieee754/dbl-64/e_log.c
parenta387428ca7330513d5f2a0924c45e6382d477052 (diff)
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Fix log (1) in round-downward mode (bug 16731).
According to ISO C Annex F, log (1) should be +0 in all rounding
modes, but some implementations in glibc wrongly return -0 in
round-downward mode (mapping to log1p (x - 1) is problematic because 1
- 1 is -0 in round-downward mode, and log1p (-0) is -0).  This patch
fixes this.  (It helps with some implementations of other functions
such as acosh, log2 and log10 that call out to log, but not enough to
enable all-rounding-modes testing for those functions without further
fixes to other implementations of them.)

Tested x86_64 and x86 and ulps updated accordingly, and did spot tests
for mips64 for the ldbl-128 fix, and i586 for the sysdeps/i386/fpu
implementations shadowed by those in sysdeps/i386/i686/fpu.

	[BZ #16731]
	* sysdeps/i386/fpu/e_log.S (__ieee754_log): Take absolute value
	when x - 1 is zero.
	* sysdeps/i386/fpu/e_logf.S (__ieee754_logf): Likewise.
	* sysdeps/i386/fpu/e_logl.S (__ieee754_logl): Likewise.
	* sysdeps/i386/i686/fpu/e_logl.S (__ieee754_logl): Likewise.
	* sysdeps/ieee754/dbl-64/e_log.c (__ieee754_log): Return +0 when
	argument is 1.
	* sysdeps/ieee754/ldbl-128/e_logl.c (__ieee754_logl): Likewise.
	* sysdeps/x86_64/fpu/e_logl.S: Take absolute value when x - 1 is
	zero.
	* math/libm-test.inc (log_test): Use ALL_RM_TEST.
	* sysdeps/i386/fpu/libm-test-ulps: Update.
	* sysdeps/x86_64/fpu/libm-test-ulps: Likewise.
Diffstat (limited to 'sysdeps/ieee754/dbl-64/e_log.c')
-rw-r--r--sysdeps/ieee754/dbl-64/e_log.c4
1 files changed, 4 insertions, 0 deletions
diff --git a/sysdeps/ieee754/dbl-64/e_log.c b/sysdeps/ieee754/dbl-64/e_log.c
index 05d318b786..4ecd3722f5 100644
--- a/sysdeps/ieee754/dbl-64/e_log.c
+++ b/sysdeps/ieee754/dbl-64/e_log.c
@@ -96,6 +96,10 @@ __ieee754_log (double x)
   if (__glibc_likely (ABS (w) > U03))
     goto case_03;
 
+  /* log (1) is +0 in all rounding modes.  */
+  if (w == 0.0)
+    return 0.0;
+
   /*--- Stage I, the case abs(x-1) < 0.03 */
 
   t8 = MHALF * w;