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Fix log2 (1) in round-downward mode (bug 17042)
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Mon, 9 Jun 2014 22:55:28 +0000
- Subject: Fix log2 (1) in round-downward mode (bug 17042)
- Authentication-results: sourceware.org; auth=none
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).
(Other functions remaining to be converted to all-rounding-mode
testing with all inputs are pow
<https://sourceware.org/ml/libc-alpha/2014-06/msg00077.html>, cexp,
cpow, jn, tgamma, yn.)
2014-06-09 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.
diff --git a/math/libm-test.inc b/math/libm-test.inc
index 0d467a2..fa8e238 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 a202bc7..73ff0ff 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 f4f9a8c..344eeb4 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 bd51b56..0f5f7e5 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 1e89284..d7424a6 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 6c7da0e..991a3b7 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 323ded0..442ad97 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 956489f..c12906d 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 bb549d2..92fef5a 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
--
Joseph S. Myers
joseph@codesourcery.com