Re: [Patch] Fix cexpl when compiled with latest GCC

[Siddhesh Poyarekar <siddhesh at sourceware dot org>]

On Friday 21 July 2017 04:31 AM, Steve Ellcey wrote:
> While testing glibc on aarch64 and building with the latest GCC (Top-of-tree)
> I got several failures involving the long double exp function.   (See
> https://sourceware.org/ml/libc-alpha/2017-07/msg00704.html)  The underflow
> exception was getting raised where it should not be.
> 
> GCC does not know that moving floating point operations around the fesetenv
> call can change the flag settings and glibc deals with this by using the
> math_force_eval and math_opt_barrier macros to restrict GCC optimizations.
> 
> This patch adds a call to math_force_eval so that the multiplication that
> creates x22 is completed before the fesetenv call and thus the underflow
> exception that is generated by the expression does not get raised (it is
> wiped out by the call to fesetenv).  That is the behavour that we want and
> what glibc is doing when compiled with older compilers.
> 
> OK to checkin?  Is this something we want in for 2.26 or should it wait
> until after the release?

This is fine for 2.26.

Siddhesh

> Steve Ellcey
> sellcey@cavium.com
> 
> 
> 2017-07-20  Steve Ellcey  <sellcey@cavium.com>
> 
> 	* sysdeps/ieee754/ldbl-128/e_expl.c (__ieee754_expl): Call
> 	math_force_eval.
> 
> 
> diff --git a/sysdeps/ieee754/ldbl-128/e_expl.c b/sysdeps/ieee754/ldbl-128/e_expl.c
> index 15639d1..fd7700c 100644
> --- a/sysdeps/ieee754/ldbl-128/e_expl.c
> +++ b/sysdeps/ieee754/ldbl-128/e_expl.c
> @@ -192,6 +192,7 @@ __ieee754_expl (_Float128 x)
>  	 with maximum error in [-2^-16-2^-53,2^-16+2^-53]
>  	 less than 4.8e-39.  */
>        x22 = x + x*x*(P1+x*(P2+x*(P3+x*(P4+x*(P5+x*P6)))));
> +      math_force_eval (x22);
>  
>        /* Return result.  */
>        fesetenv (&oldenv);
>