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Re: The state of glibc libm

On 2012-03-26 11:15:37 -0500, Steven Munroe wrote:
> On Mon, 2012-03-26 at 12:26 +0200, Vincent Lefevre wrote:
> > Then concerning double-double vs quad (binary128) for the "long double"
> > type, I think that quad would be more useful, in particular because
> > it has been standardized and it is a true FP format. If need be (for
> > efficiency reasons), double-double could still be implemented using
> > the "double" type, via a library or ad-hoc code (that does something
> > more clever, taking the context into account). And the same code (with
> > just a change of the base type) could be reused to get a double-quad
> > (i.e. quad + quad) arithmetic, that can be useful to implement the
> > "long double" versions of the math functions (expl, and so on).
> > 
> This is much easier said then done. In practice it is a major ABI change
> and would have to be staged over multiple (7-10) years.

Yes, I meant in the long term.

> > > > So, in the long term, the ABI should probably be changed to have
> > > > long double = quadruple precision (binary128).
> > > 
> > > The ABI for Power Architecture changed away from quad precision to using 
> > > IBM long double (the original SysV ABI for PowerPC used quad precision, 
> > > the current ABI uses IBM long double)....
> > 
> > Perhaps they could change back to quad precision.
> > 
> That is not the feedback we get from our customers. No one will use
> software IEEE binary128 and we don't have hardware binary128. So far
> there is abstract interest but no strong demand for this. So there is no
> incentive to change.

But perhaps one reason why there is no hardware binary128 is that
there is no software that uses it.

BTW, what is the feedback from your customers concerning the
long double math functions (expl, etc.)?

Vincent Lefèvre <> - Web: <>
100% accessible validated (X)HTML - Blog: <>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

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