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.

12 files changed, 88 insertions, 9 deletions

diff --git a/ChangeLog b/ChangeLog
index f8f87bdb06..e3601495d0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,20 @@
+2014-03-21  Joseph Myers  <joseph@codesourcery.com>
+
+	[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.
+
 2014-03-21  Siddhesh Poyarekar  <siddhesh@redhat.com>
 
 	* scripts/bench.pl: Remove file.
diff --git a/NEWS b/NEWS
index 5f9ffcc569..5df0277bee 100644
--- a/NEWS
+++ b/NEWS
@@ -11,7 +11,7 @@ Version 2.20
 
   15347, 15804, 15894, 16447, 16532, 16545, 16574, 16600, 16609, 16610,
   16611, 16613, 16623, 16632, 16639, 16642, 16649, 16670, 16674, 16677,
-  16680, 16683, 16689, 16695, 16701, 16706, 16707.
+  16680, 16683, 16689, 16695, 16701, 16706, 16707, 16731.
 
 * Running the testsuite no longer terminates as soon as a test fails.
   Instead, a file tests.sum (xtests.sum from "make xcheck") is generated,
diff --git a/math/libm-test.inc b/math/libm-test.inc
index 7abc7c14b5..5e50f0ee3d 100644
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -7786,9 +7786,7 @@ static const struct test_f_f_data log_test_data[] =
 static void
 log_test (void)
 {
-  START (log, 0);
-  RUN_TEST_LOOP_f_f (log, log_test_data, );
-  END;
+  ALL_RM_TEST (log, 0, log_test_data, RUN_TEST_LOOP_f_f, END);
 }
 
 
diff --git a/sysdeps/i386/fpu/e_log.S b/sysdeps/i386/fpu/e_log.S
index 0877924998..3fa32aad3c 100644
--- a/sysdeps/i386/fpu/e_log.S
+++ b/sysdeps/i386/fpu/e_log.S
@@ -46,7 +46,13 @@ ENTRY(__ieee754_log)
 	fnstsw			// x-1 : x : log(2)
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : log(2)
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : log(2)
 	fyl2xp1			// log(x)
 	ret
 
diff --git a/sysdeps/i386/fpu/e_logf.S b/sysdeps/i386/fpu/e_logf.S
index 485180e909..ca83d39cef 100644
--- a/sysdeps/i386/fpu/e_logf.S
+++ b/sysdeps/i386/fpu/e_logf.S
@@ -47,7 +47,13 @@ ENTRY(__ieee754_logf)
 	fnstsw			// x-1 : x : log(2)
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : log(2)
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : log(2)
 	fyl2xp1			// log(x)
 	ret
 
diff --git a/sysdeps/i386/fpu/e_logl.S b/sysdeps/i386/fpu/e_logl.S
index d7a459a627..edae1d70df 100644
--- a/sysdeps/i386/fpu/e_logl.S
+++ b/sysdeps/i386/fpu/e_logl.S
@@ -47,7 +47,13 @@ ENTRY(__ieee754_logl)
 	fnstsw			// x-1 : x : log(2)
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : log(2)
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : log(2)
 	fyl2xp1			// log(x)
 	ret
 
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
diff --git a/sysdeps/i386/i686/fpu/e_logl.S b/sysdeps/i386/i686/fpu/e_logl.S
index 8a86222b13..a0d1107dc0 100644
--- a/sysdeps/i386/i686/fpu/e_logl.S
+++ b/sysdeps/i386/i686/fpu/e_logl.S
@@ -46,7 +46,13 @@ ENTRY(__ieee754_logl)
 	fcomip	%st(1)		// |x-1| : x-1 : x : log(2)
 	fstp	%st(0)		// x-1 : x : log(2)
 	jc	2f
-	fstp	%st(1)		// x-1 : log(2)
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	4f
+	fabs			// log(1) is +0 in all rounding modes.
+4:	fstp	%st(1)		// x-1 : log(2)
 	fyl2xp1			// log(x)
 	ret
 
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;
diff --git a/sysdeps/ieee754/ldbl-128/e_logl.c b/sysdeps/ieee754/ldbl-128/e_logl.c
index 3d1034dd61..cb43816793 100644
--- a/sysdeps/ieee754/ldbl-128/e_logl.c
+++ b/sysdeps/ieee754/ldbl-128/e_logl.c
@@ -240,6 +240,8 @@ __ieee754_logl(long double x)
   /* On this interval the table is not used due to cancellation error.  */
   if ((x <= 1.0078125L) && (x >= 0.9921875L))
     {
+      if (x == 1.0L)
+	return 0.0L;
       z = x - 1.0L;
       k = 64;
       t.value  = 1.0L;
diff --git a/sysdeps/x86_64/fpu/e_logl.S b/sysdeps/x86_64/fpu/e_logl.S
index a8e31084ba..315afc0033 100644
--- a/sysdeps/x86_64/fpu/e_logl.S
+++ b/sysdeps/x86_64/fpu/e_logl.S
@@ -45,7 +45,13 @@ ENTRY(__ieee754_logl)
 	fnstsw			// x-1 : x : log(2)
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : log(2)
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : log(2)
 	fyl2xp1			// log(x)
 	ret
 
diff --git a/sysdeps/x86_64/fpu/libm-test-ulps b/sysdeps/x86_64/fpu/libm-test-ulps
index 5f4ab06050..301eaa6542 100644
--- a/sysdeps/x86_64/fpu/libm-test-ulps
+++ b/sysdeps/x86_64/fpu/libm-test-ulps
@@ -1170,6 +1170,22 @@ ifloat: 1
 ildouble: 1
 ldouble: 1
 
+Function: "log_downward":
+float: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
+Function: "log_towardzero":
+ildouble: 1
+ldouble: 1
+
+Function: "log_upward":
+float: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
 Function: "pow":
 float: 1
 ifloat: 1