[patch, libstdc++] std::shuffle: Generate two swap positions at a time if possible

[Eelis]

>>> On 2016-05-01 16:18, Eelis wrote:
>>>> The attached patch optimizes std::shuffle for the very common case
>>>> where the generator range is large enough that a single invocation
>>>> can produce two swap positions.
>>>>
>>>> This reduces the runtime of the following testcase by 37% on my machine:

Gentle ping. :)  Did anyone get a chance to take a look at this?

Does the idea seem sound? Does the implementation seem correct?

Thanks,

Eelis