git: 93fc67896550 - main - Fix incorrect powf(3) result with x near 1 and |y| much larger than 1

[Alexander Richardson]

On Mon, 8 Feb 2021, 19:49 Dimitry Andric, <dim at freebsd.org> wrote:

> The branch main has been updated by dim:
>
> URL:
> https://cgit.FreeBSD.org/src/commit/?id=93fc67896550548f91b307dbe3053f11db5d4a8a
>
> commit 93fc67896550548f91b307dbe3053f11db5d4a8a
> Author:     Steve Kargl <sgk at troutmask.apl.washington.edu>
> AuthorDate: 2021-02-08 19:45:30 +0000
> Commit:     Dimitry Andric <dim at FreeBSD.org>
> CommitDate: 2021-02-08 19:48:43 +0000
>
>     Fix incorrect powf(3) result with x near 1 and |y| much larger than 1
>
>     This adjusts the check to trigger overflow/underflow to a slightly
> lower
>     value.
>
>     Before: powf(9.999995e-01, -1.342177e+08) -> inf
>     After:  powf(9.999995e-01, -1.342177e+08) -> 1.858724e+31
>
>     MFC after:      1 week
> ---
>  lib/msun/src/e_powf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/msun/src/e_powf.c b/lib/msun/src/e_powf.c
> index 53f1d37d6bec..33eedad50b16 100644
> --- a/lib/msun/src/e_powf.c
> +++ b/lib/msun/src/e_powf.c
> @@ -136,7 +136,7 @@ __ieee754_powf(float x, float y)
>      /* |y| is huge */
>         if(iy>0x4d000000) { /* if |y| > 2**27 */
>         /* over/underflow if x is not close to one */
> -           if(ix<0x3f7ffff8) return (hy<0)? sn*huge*huge:sn*tiny*tiny;
> +           if(ix<0x3f7ffff7) return (hy<0)? sn*huge*huge:sn*tiny*tiny;
>             if(ix>0x3f800007) return (hy>0)? sn*huge*huge:sn*tiny*tiny;
>         /* now |1-x| is tiny <= 2**-20, suffice to compute
>            log(x) by x-x^2/2+x^3/3-x^4/4 */
>


Do we have a test with previously buggy values? If not it would be good to
add one.

Alex

>