Adding __float128 (i.e TS 18661-3)

[Steven Munroe]

On Wed, 2016-06-08 at 12:33 +0000, Joseph Myers wrote:
> On Tue, 7 Jun 2016, Steven Munroe wrote:
> 
> > And we would like to avoid compile errors due to missing compiler
> > options and associated type randomization.
> 
> Compile errors are fine - they make the problem obvious.  What should be 
> avoided is cases where code quietly gets built with the wrong type without 
> producing errors, but does the wrong thing at runtime.  (And there are a 
> range of possible solutions to avoid that, including e.g. #error for the 
> wrong long double format in a header used in the ldbl-128ibm code.)
> 
There are cases were the compiler will silently convert types (at great
expense in performance) and not tell you anything.

The only way to find these is to look at the generated code. I found
several of these in my early prototypes for this work.

> > Separately we do use the printf plugin API to support _Decimal Float and
> > vector printf and provide these to Linux on Power customers. It is fair
> > to assume that we would provide a 'Q' format plugin for early adopters
> > of __float128. 
> 
> Sure, and if people want to use scanf like that I think it would be 
> reasonable to add scanf hooks support similar to the printf hooks support.
>