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Re: [PATCH][BZ #16824] Fix failing y1 due to too large ulps in downward/upward rounding mode.
- From: Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>
- To: libc-alpha at sourceware dot org
- Date: Fri, 11 Apr 2014 09:50:23 -0300
- Subject: Re: [PATCH][BZ #16824] Fix failing y1 due to too large ulps in downward/upward rounding mode.
- Authentication-results: sourceware.org; auth=none
- References: <li8la8$llb$1 at ger dot gmane dot org>
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.