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

[Stefan Liebler]

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.
---

-------------- next part --------------
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;
     }