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.

1 files changed, 12 insertions, 0 deletions

diff --git a/sysdeps/i386/fpu/libm-test-ulps b/sysdeps/i386/fpu/libm-test-ulps
index 3be1806826..b7b2e12b54 100644
--- a/sysdeps/i386/fpu/libm-test-ulps
+++ b/sysdeps/i386/fpu/libm-test-ulps
@@ -1096,6 +1096,18 @@ Function: "log1p":
 ildouble: 1
 ldouble: 1
 
+Function: "log_downward":
+ildouble: 1
+ldouble: 1
+
+Function: "log_towardzero":
+ildouble: 1
+ldouble: 1
+
+Function: "log_upward":
+ildouble: 1
+ldouble: 1
+
 Function: "pow":
 ildouble: 1
 ldouble: 1