Re: [patch] pow.3: Add note on performance characteristics of pow

["Michael Kerrisk (man-pages)" <mtk dot manpages at gmail dot com>]

Hello Manuel,


On 10/17/2014 01:51 AM, Manuel López-Ibáñez wrote:
> Hello,
> 
> This patch documents
> https://sourceware.org/bugzilla/show_bug.cgi?id=13932 as requested in
> https://sourceware.org/bugzilla/show_bug.cgi?id=15267. It would be
> nice to add more details, but it seems the behavior and its
> consequences is not well understood. The purpose of the note is to
> warn users of this behavior and thus avoid frustration
> (http://entropymine.com/imageworsener/slowpow/).
> 
> This is my first patch. I followed (hopefully correctly) the instructions here:
> https://www.kernel.org/doc/man-pages/patches.html


Thanks for this patch. It looks good. But inline patches are easier to review.
One small question below.

> diff --git a/man3/pow.3 b/man3/pow.3
> index d0a2ea8..3d2151a 100644
> --- a/man3/pow.3
> +++ b/man3/pow.3
> @@ -320,6 +320,17 @@ The variant returning
>  also conforms to
>  SVr4, 4.3BSD, C89.
>  .SH BUGS
> +On 64-bits,
> +.\"
> +.\" https://sourceware.org/bugzilla/show_bug.cgi?id=13932
> +.BR pow ()
> +may be more than 10,000 times slower for some inputs than for other nearby

I plan to change "some" to "some (rare)". Sound okay? The point is not to
worry people that this is a problem that is commonly encountered. (Or is it?)

Thanks,

Michael

> +inputs. The inputs that produce such behavior are different for
> +.BR powf (),
> +.BR pow ()
> +and
> +.BR powl ().
> +
>  In glibc 2.9 and earlier,
>  .\"
>  .\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=6776


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/