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Fix log (1) in round-downward mode (bug 16731)
- From: "Joseph S. Myers" <joseph at codesourcery dot com>
- To: <libc-alpha at sourceware dot org>
- Date: Fri, 21 Mar 2014 17:47:45 +0000
- Subject: Fix log (1) in round-downward mode (bug 16731)
- Authentication-results: sourceware.org; auth=none
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.
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.
diff --git a/math/libm-test.inc b/math/libm-test.inc
index 7abc7c1..5e50f0e 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 0877924..3fa32aa 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 485180e..ca83d39 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 d7a459a..edae1d7 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 3be1806..b7b2e12 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 8a86222..53a3d10 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 05d318b..4ecd372 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 3d1034d..cb43816 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 a8e3108..315afc0 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 5f4ab06..301eaa6 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
--
Joseph S. Myers
joseph@codesourcery.com