[PATCH][BZ #16824] Fix failing y1 due to too large ulps in downward/upward rounding mode.

[Stefan Liebler]

Thanks for the hint with SET_RESTORE_ROUND macro.
Here is the updated version.
Tested on s390/s390x.

Ok for commit?

Bye

---
2014-04-14  Stefan Liebler  <stli@linux.vnet.ibm.com>

	[BZ #16824]
	* sysdeps/ieee754/ldbl-128/e_j1l.c (__ieee754_y1l):
	Set round-to-nearest internally to reduce error accumulation.
---

On 04/11/2014 02:50 PM, Adhemerval Zanella wrote:
> On 11-04-2014 08:56, Stefan Liebler wrote:
>> Hi,
>>
>> on s390 the y1 tests for long double fails due to too large ulps in downward / upward rounding mode (See Bug 16824).
>> According to Joseph Myers patch comment
>> (https://sourceware.org/ml/libc-alpha/2014-03/msg00620.html),
>> round-to-nearest is used internally to reduce error accumulation.
>> Tests on s390/s390x shows following ulps:
>> y1: 2
>> y1_downward: 4
>> y1_towardzero: 2
>> y1_upward: 5
>>
>> Is this okay? On other architectures too?
>>
>> Bye
>> ---
>> 2014-04-11  Stefan Liebler  <stli@linux.vnet.ibm.com>
>>
>>      [BZ #16824]
>>      * sysdeps/ieee754/ldbl-128/e_j1l.c (__ieee754_y1l):
>>      Set round-to-nearest internally to reduce error accumulation.
>> ---
>>
>> diff --git a/sysdeps/ieee754/ldbl-128/e_j1l.c b/sysdeps/ieee754/ldbl-128/e_j1l.c
>> index 70a1c86..db96c94 100644
>> --- a/sysdeps/ieee754/ldbl-128/e_j1l.c
>> +++ b/sysdeps/ieee754/ldbl-128/e_j1l.c
>> @@ -855,11 +855,14 @@ __ieee754_y1l (long double x)
>>       return -TWOOPI / x;
>>     if (xx <= 2.0L)
>>       {
>> +      int save_round = fegetround ();
>> +      libc_fesetround (FE_TONEAREST);
>>         /* 0 <= x <= 2 */
>>         z = xx * xx;
>>         p = xx * neval (z, Y0_2N, NY0_2N) / deval (z, Y0_2D, NY0_2D);
>>         p = -TWOOPI / xx + p;
>>         p = TWOOPI * __ieee754_logl (x) * __ieee754_j1l (x) + p;
>> +      libc_fesetround (save_round);
>>         return p;
>>       }
>
> Although s390 does not implemented the optimization to set/restore rounding mode only
> when needed (check the x86_64 and powerpc fenv_private.h), the way to set/reset
> rounding mode should using SET_RESTORE_ROUND.
>
>

-------------- next part --------------
diff --git a/sysdeps/ieee754/ldbl-128/e_j1l.c b/sysdeps/ieee754/ldbl-128/e_j1l.c
index 70a1c86..1264c95 100644
--- a/sysdeps/ieee754/ldbl-128/e_j1l.c
+++ b/sysdeps/ieee754/ldbl-128/e_j1l.c
@@ -856,6 +856,7 @@ __ieee754_y1l (long double x)
   if (xx <= 2.0L)
     {
       /* 0 <= x <= 2 */
+      SET_RESTORE_ROUNDL (FE_TONEAREST);
       z = xx * xx;
       p = xx * neval (z, Y0_2N, NY0_2N) / deval (z, Y0_2D, NY0_2D);
       p = -TWOOPI / xx + p;