[Cfp-interest 2853] Re: CFP review of TS-4 and TS-5 revisions

[David Hough CFP]

Exactly what the representable set of and bit encoding of double-double are
unstandardized and I suppose most implementations are "whatever works
fastest".    If speed if no concern, then just use software quad.

The short algorithms I listed do have the property that {max:max} is
not a possible result, and indeed the final result {x.h;x.t} will always
have |x.h| > |x.t|.    Whether the signs agree might indeed depend on the
rounding mode.   754r however says that the augmented operations compute
x.h using roundTiesTowardZero.   So the signs should always agree.