Fix log2 (1) in round-downward mode (bug 17042).

As with other issues of this kind, bug 17042 is log2 (1) wrongly
returning -0 instead of +0 in round-downward mode because of
implementations effectively in terms of log1p (x - 1).  This patch
fixes the issue in the same way used for log and log10.

Tested x86_64 and x86 and ulps updated accordingly.  Also tested for
mips64 to confirm a fix was needed for ldbl-128 and to validate that
fix (also applied to ldbl-128ibm since that version of log2l is
essentially the same as the ldbl-128 one).

	[BZ #17042]
	* sysdeps/i386/fpu/e_log2.S (__ieee754_log2): Take absolete value
	when x - 1 is zero.
	* sysdeps/i386/fpu/e_log2f.S (__ieee754_log2f): Likewise.
	* sysdeps/i386/fpu/e_log2l.S (__ieee754_log2l): Likewise.
	* sysdeps/ieee754/ldbl-128/e_log2l.c (__ieee754_log2l): Return
	0.0L for an argument of 1.0L.
	* sysdeps/ieee754/ldbl-128ibm/e_log2l.c (__ieee754_log2l):
	Likewise.
	* sysdeps/x86_64/fpu/e_log2l.S (__ieee754_log2l): Take absolute
	value when x - 1 is zero.
	* math/libm-test.inc (log2_test): Use ALL_RM_TEST.
	* sysdeps/i386/fpu/libm-test-ulps: Update.
	* sysdeps/x86_64/fpu/libm-test-ulps: Likewise.

11 files changed, 91 insertions, 8 deletions

diff --git a/ChangeLog b/ChangeLog
index 0ace298b9e..9055159cf0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,20 @@
+2014-06-10  Joseph Myers  <joseph@codesourcery.com>
+
+	[BZ #17042]
+	* sysdeps/i386/fpu/e_log2.S (__ieee754_log2): Take absolete value
+	when x - 1 is zero.
+	* sysdeps/i386/fpu/e_log2f.S (__ieee754_log2f): Likewise.
+	* sysdeps/i386/fpu/e_log2l.S (__ieee754_log2l): Likewise.
+	* sysdeps/ieee754/ldbl-128/e_log2l.c (__ieee754_log2l): Return
+	0.0L for an argument of 1.0L.
+	* sysdeps/ieee754/ldbl-128ibm/e_log2l.c (__ieee754_log2l):
+	Likewise.
+	* sysdeps/x86_64/fpu/e_log2l.S (__ieee754_log2l): Take absolute
+	value when x - 1 is zero.
+	* math/libm-test.inc (log2_test): Use ALL_RM_TEST.
+	* sysdeps/i386/fpu/libm-test-ulps: Update.
+	* sysdeps/x86_64/fpu/libm-test-ulps: Likewise.
+
 2014-06-09  Bernard Ogden  <bernie.ogden@linaro.org>
 
 	[BZ #15119]
diff --git a/NEWS b/NEWS
index 622cdbf21d..ca3ef633b0 100644
--- a/NEWS
+++ b/NEWS
@@ -19,7 +19,7 @@ Version 2.20
   16791, 16796, 16799, 16800, 16815, 16823, 16824, 16831, 16838, 16849,
   16854, 16876, 16877, 16878, 16882, 16885, 16888, 16890, 16912, 16915,
   16916, 16917, 16922, 16927, 16928, 16932, 16943, 16958, 16965, 16966,
-  16967, 16977, 16978, 16984, 16990, 17009.
+  16967, 16977, 16978, 16984, 16990, 17009, 17042.
 
 * The minimum Linux kernel version that this version of the GNU C Library
   can be used with is 2.6.32.
diff --git a/math/libm-test.inc b/math/libm-test.inc
index 0d467a2195..fa8e2385dc 100644
--- a/math/libm-test.inc
+++ b/math/libm-test.inc
@@ -7840,9 +7840,7 @@ static const struct test_f_f_data log2_test_data[] =
 static void
 log2_test (void)
 {
-  START (log2, 0);
-  RUN_TEST_LOOP_f_f (log2, log2_test_data, );
-  END;
+  ALL_RM_TEST (log2, 0, log2_test_data, RUN_TEST_LOOP_f_f, END);
 }
 
 
diff --git a/sysdeps/i386/fpu/e_log2.S b/sysdeps/i386/fpu/e_log2.S
index a202bc731d..73ff0fffd3 100644
--- a/sysdeps/i386/fpu/e_log2.S
+++ b/sysdeps/i386/fpu/e_log2.S
@@ -47,7 +47,13 @@ ENTRY(__ieee754_log2)
 	fnstsw			// x-1 : x : 1
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : 1
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log2(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : 1
 	fyl2xp1			// log(x)
 	ret
 
diff --git a/sysdeps/i386/fpu/e_log2f.S b/sysdeps/i386/fpu/e_log2f.S
index f4f9a8c3bf..344eeb495e 100644
--- a/sysdeps/i386/fpu/e_log2f.S
+++ b/sysdeps/i386/fpu/e_log2f.S
@@ -47,7 +47,13 @@ ENTRY(__ieee754_log2f)
 	fnstsw			// x-1 : x : 1
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : 1
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log2(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : 1
 	fyl2xp1			// log(x)
 	ret
 
diff --git a/sysdeps/i386/fpu/e_log2l.S b/sysdeps/i386/fpu/e_log2l.S
index bd51b5651e..0f5f7e5789 100644
--- a/sysdeps/i386/fpu/e_log2l.S
+++ b/sysdeps/i386/fpu/e_log2l.S
@@ -47,7 +47,13 @@ ENTRY(__ieee754_log2l)
 	fnstsw			// x-1 : x : 1
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : 1
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log2(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : 1
 	fyl2xp1			// log(x)
 	ret
 
diff --git a/sysdeps/i386/fpu/libm-test-ulps b/sysdeps/i386/fpu/libm-test-ulps
index 1e89284455..d7424a6f86 100644
--- a/sysdeps/i386/fpu/libm-test-ulps
+++ b/sysdeps/i386/fpu/libm-test-ulps
@@ -1588,6 +1588,22 @@ ifloat: 1
 ildouble: 1
 ldouble: 1
 
+Function: "log2_towardzero":
+double: 1
+float: 1
+idouble: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
+Function: "log2_upward":
+double: 1
+float: 1
+idouble: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
 Function: "log_downward":
 ildouble: 1
 ldouble: 1
diff --git a/sysdeps/ieee754/ldbl-128/e_log2l.c b/sysdeps/ieee754/ldbl-128/e_log2l.c
index 6c7da0e44b..991a3b73e2 100644
--- a/sysdeps/ieee754/ldbl-128/e_log2l.c
+++ b/sysdeps/ieee754/ldbl-128/e_log2l.c
@@ -188,6 +188,9 @@ __ieee754_log2l (x)
   if (hx >= 0x7fff000000000000LL)
     return (x + x);
 
+  if (x == 1.0L)
+    return 0.0L;
+
 /* separate mantissa from exponent */
 
 /* Note, frexp is used so that denormal numbers
diff --git a/sysdeps/ieee754/ldbl-128ibm/e_log2l.c b/sysdeps/ieee754/ldbl-128ibm/e_log2l.c
index 323ded0c0f..442ad97254 100644
--- a/sysdeps/ieee754/ldbl-128ibm/e_log2l.c
+++ b/sysdeps/ieee754/ldbl-128ibm/e_log2l.c
@@ -190,6 +190,9 @@ __ieee754_log2l (x)
   if (hx >= 0x7ff0000000000000LL)
     return (x + x);
 
+  if (x == 1.0L)
+    return 0.0L;
+
 /* separate mantissa from exponent */
 
 /* Note, frexp is used so that denormal numbers
diff --git a/sysdeps/x86_64/fpu/e_log2l.S b/sysdeps/x86_64/fpu/e_log2l.S
index 956489fc3e..c12906d456 100644
--- a/sysdeps/x86_64/fpu/e_log2l.S
+++ b/sysdeps/x86_64/fpu/e_log2l.S
@@ -45,7 +45,13 @@ ENTRY(__ieee754_log2l)
 	fnstsw			// x-1 : x : 1
 	andb	$0x45, %ah
 	jz	2f
-	fstp	%st(1)		// x-1 : 1
+	fxam
+	fnstsw
+	andb	$0x45, %ah
+	cmpb	$0x40, %ah
+	jne	5f
+	fabs			// log2(1) is +0 in all rounding modes.
+5:	fstp	%st(1)		// x-1 : 1
 	fyl2xp1			// log(x)
 	ret
 
diff --git a/sysdeps/x86_64/fpu/libm-test-ulps b/sysdeps/x86_64/fpu/libm-test-ulps
index bb549d2b0d..92fef5a134 100644
--- a/sysdeps/x86_64/fpu/libm-test-ulps
+++ b/sysdeps/x86_64/fpu/libm-test-ulps
@@ -1665,6 +1665,28 @@ ifloat: 1
 ildouble: 1
 ldouble: 1
 
+Function: "log2_downward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+
+Function: "log2_towardzero":
+double: 1
+float: 1
+idouble: 1
+ifloat: 1
+ildouble: 1
+ldouble: 1
+
+Function: "log2_upward":
+double: 2
+float: 2
+idouble: 2
+ifloat: 2
+ildouble: 1
+ldouble: 1
+
 Function: "log_downward":
 float: 1
 ifloat: 1