Re: [patch] std::experimental::gcd and std::experimental::lcd

[Jonathan Wakely <jwakely at redhat dot com>]

On 02/05/15 18:27 +0200, Marc Glisse wrote:
> On Sat, 2 May 2015, Jonathan Wakely wrote:
>
> > These where simple to implement (almost too simple ... I probably
> > got something wrong!)
>
> I didn't remember that std::abs works for unsigned. It will need more 
> work for performance, but that can certainly be done later (I didn't 
> look at the code beyond checking what you meant by "simple").

std::abs seems to work fine for unsigned, the overload in <cmath> for
integral types just uses __builtin_fabs. Maybe it would be better for
gcd() to just use that directly instead of including <cmath> (as
attached, which also removes the qualification on the call to gcd
because the functions only work for integral types which have no
associated namespaces anyway).

> > (Apart from using common_type_t, which is easy to change, these
> > functions meet the simpler rules for C++11 constexpr, so moving them
> > out of <experimental/numeric> would probably allow <ratio> to be
> > greatly simplified. I don't plan on doing that myself any time soon,
> > but it would make sense to do it some day.)
>
> gcd is not really the hard part in ratio. But constexpr should help, 
> it made sense not to have it in tr1, but I don't remember why we 
> didn't use it in the more recent changes (2011, the compiler probably 
> already supported constexpr). Maybe the interesting functions were too 
> hard to write as one-liners...

<ratio> was added to libstdc++ in 2008 so I think the constexpr
support was not good enough at the time.

Attachment: abs.txt
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