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Re: Fix e_logl (128ibm) spurious underflow
- From: Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>
- To: libc-alpha at sourceware dot org
- Date: Thu, 28 Feb 2013 02:02:52 -0300
- Subject: Re: Fix e_logl (128ibm) spurious underflow
- References: <512E686B.9040007@linux.vnet.ibm.com> <Pine.LNX.4.64.1302272347150.18831@digraph.polyomino.org.uk>
On 02/27/2013 08:50 PM, Joseph S. Myers wrote:
> On Wed, 27 Feb 2013, Adhemerval Zanella wrote:
>
>> 2013-02-27 Adhemerval Zanella <azanella@linux.vnet.ibm.com>
>>
>> * sysdeps/ieee754/ldbl-128ibm/e_logl.c (__ieee754_logl): Fix spurious
>> underflow.
>> * sysdeps/powerpc/fpu/libm-test-ulps: Update.
> What is the bug report in Bugzilla for this issue, which presumably is
> user-visible in a release so should have a bug report? If there isn't
> one, please file one; in any case, include the [BZ #N] annotation in the
> ChangeLog entry (and a commit of a fix should include updating NEWS).
> (If the issue isn't user-visible in a release and so doesn't need a bug
> report, please explain why not. But any bug being fixed that was
> user-visible in a release should have a corresponding Bugzilla entry.)
>
> In my view you should add a testcase for this issue for log to
> libm-test.inc, even if there are tests for other functions that show the
> problem as well.
>
I create BZ #15214 to track this issue. I didn't add the specific testcase yet
because it is showing some higher ULPs that I'd like to take a look (this is
showing without my patch also).
--
2013-02-28 Adhemerval Zanella <azanella@linux.vnet.ibm.com>
[BZ #15214]
* NEWS: Mention BZ #15214.
* sysdeps/ieee754/ldbl-128ibm/e_logl.c (__ieee754_logl): Fix spurious
underflow.
* sysdeps/powerpc/fpu/libm-test-ulps: Update.
diff --git a/NEWS b/NEWS
index a4c5638..945aa9d 100644
--- a/NEWS
+++ b/NEWS
@@ -11,7 +11,7 @@ Version 2.18
11561, 13951, 14142, 14200, 14317, 14327, 14496, 14920, 14964, 14981,
14982, 14985, 14994, 14996, 15003, 15006, 15020, 15023, 15036, 15054,
- 15062, 15078, 15160.
+ 15062, 15078, 15160, 15214.
* Add support for calling C++11 thread_local object destructors on thread
and program exit. This needs compiler support for offloading C++11
diff --git a/sysdeps/ieee754/ldbl-128ibm/e_logl.c b/sysdeps/ieee754/ldbl-128ibm/e_logl.c
index 14f47eb..15b5edf 100644
--- a/sysdeps/ieee754/ldbl-128ibm/e_logl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/e_logl.c
@@ -182,6 +182,9 @@ static const long double
ln2a = 6.93145751953125e-1L,
ln2b = 1.4286068203094172321214581765680755001344E-6L;
+static const long double
+ ldbl_epsilon = 0x1p-106L;
+
long double
__ieee754_logl(long double x)
{
@@ -258,7 +261,12 @@ __ieee754_logl(long double x)
}
/* Series expansion of log(1+z). */
w = z * z;
- y = ((((((((((((l15 * z
+ /* Avoid spurious underflows. */
+ if (__glibc_unlikely(w <= ldbl_epsilon))
+ y = 0.0L;
+ else
+ {
+ y = ((((((((((((l15 * z
+ l14) * z
+ l13) * z
+ l12) * z
@@ -271,7 +279,8 @@ __ieee754_logl(long double x)
+ l5) * z
+ l4) * z
+ l3) * z * w;
- y -= 0.5 * w;
+ y -= 0.5 * w;
+ }
y += e * ln2b; /* Base 2 exponent offset times ln(2). */
y += z;
y += logtbl[k-26]; /* log(t) - (t-1) */
diff --git a/sysdeps/powerpc/fpu/libm-test-ulps b/sysdeps/powerpc/fpu/libm-test-ulps
index 4221967..5072190 100644
--- a/sysdeps/powerpc/fpu/libm-test-ulps
+++ b/sysdeps/powerpc/fpu/libm-test-ulps
@@ -243,9 +243,11 @@ idouble: 1
Test "Real part of: cacos (-0.5 + +0 i) == 2.094395102393195492308428922186335256131 - 0 i":
double: 1
idouble: 1
+ldouble: 1
Test "Real part of: cacos (-0.5 - 0 i) == 2.094395102393195492308428922186335256131 + +0 i":
double: 1
idouble: 1
+ldouble: 1
Test "Imaginary part of: cacos (-1.5 + +0 i) == pi - 0.9624236501192068949955178268487368462704 i":
double: 1
float: 1
@@ -265,6 +267,10 @@ double: 1
float: 1
idouble: 1
ifloat: 1
+Test "Imaginary part of: cacos (0x1.fp1023 + 0x1.fp1023 i) == 7.853981633974483096156608458198757210493e-1 - 7.107906849659093345062145442726115449315e2 i":
+ldouble: 1
+Test "Imaginary part of: cacos (0x1.fp127 + 0x1.fp127 i) == 7.853981633974483096156608458198757210493e-1 - 8.973081118419833726837456344608533993585e1 i":
+ldouble: 1
# cacosh
Test "Real part of: cacosh (+0 + 0.5 i) == 0.4812118250596034474977589134243684231352 + pi/2 i":
--
1.7.1