Re: Fix e_logl (128ibm) spurious underflow

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

Ping.


On 03/07/2013 02:33 PM, Adhemerval Zanella wrote:
> Ping.
>
> On 02/28/2013 02:02 AM, Adhemerval Zanella wrote:
>> 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":