Accurate summation algorithm

[Tamas_Papp]

@stevengj

As I commented elsewhere, if -ffast-math dramatically speeds up some code, we can ask what algebraic transformation it did that helped. We can then decide whether to do that transformation manually (in all languages).

I think that a blog post (or a series) by someone qualified about these issues would really help. Despite reading Goldberg (1991) and a couple of similar papers and book chapters, my standard method for learning about these issues is getting burned by them.