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[Bug math/13932] x86_64 pow unexpectedly slow for some inputs
- From: "siddhesh at redhat dot com" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sourceware dot org
- Date: Mon, 11 Mar 2013 10:19:19 +0000
- Subject: [Bug math/13932] x86_64 pow unexpectedly slow for some inputs
- Auto-submitted: auto-generated
- References: <bug-13932-131@http.sourceware.org/bugzilla/>
http://sourceware.org/bugzilla/show_bug.cgi?id=13932
--- Comment #8 from Siddhesh Poyarekar <siddhesh at redhat dot com> 2013-03-11 10:19:19 UTC ---
(In reply to comment #7)
> Hi, thanks for your work. Personally, I would be interested to have a pow
> version that is perhaps less precise but that does not have such nasty
> corner-cases. A 4x improvement is nice, but given that the worst-case may be
> 12,000x slower, it is not a definitive solution.
>
> There are some implementations on the net that perhaps could be used when
> -ffast-math is given?
>
> http://jrfonseca.blogspot.be/2008/09/fast-sse2-pow-tables-or-polynomials.html
>
> http://www.dctsystems.co.uk/Software/power.html
>
> http://martin.ankerl.com/2012/01/25/optimized-approximative-pow-in-c-and-cpp/
>
> I am no expert in IEEE floating-point arithmetic, so it may well be that those
> implementations produce extremely bad approximations for some inputs. Is that
> the case?
They're all in a way subsets of what the fast phase of libm does, so we don't
really need to reimplement libm to take advantage of those ideas.
That said, there is indeed a strong case for fast and (for some inputs)
imprecise computations of transcendentals. That probably won't be the default
in glibc, but I won't rule out the possibility of having an alternate library
or configuration, provided there is consensus in the glibc community for it.
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