Implement C _FloatN, _FloatNx types

[Joseph Myers]

On Tue, 21 Jun 2016, Bill Schmidt wrote:

> I haven't read through the patch in detail yet, but thank you for the 
> very thorough description!  For PowerPC, we currently have a lot 
> invested in having __float128 correspond to KFmode for the time being, 
> during the transition while supporting both IEEE-128 and IBM long 
> double.  (I was not aware of the problem with q-suffix constants being 
> treated as TFmode, which will need to be fixed.)  I'm currently in 
> process of developing the minimal set of *f128 built-ins required for 
> glibc, using __float128 and KFmode.
> 
> Can we please keep this correspondence for now?  Do you foresee any 
> further concerns with staying on that path until we can fully throw the 
> switch to move away from IBM long double?

The question is what mode is used for _Float128 (and thus for __float128, 
which is now a typedef for _Float128) in the FLOAT128_IEEE_P (TFmode) case 
(that is, when TARGET_IEEEQUAD is true).  I see no problems with that mode 
being KFmode instead of TFmode if that's what's preferred; I'll make that 
change in the next revision of my patch.  Of course, you should make *q 
constants match, so that they are consistent with *f128 constants.

-- 
Joseph S. Myers
joseph@codesourcery.com