Re: The state of glibc libm

[Vincent Lefevre <vincent+gcc at vinc17 dot org>]

On 2012-03-22 16:29:00 +0000, Joseph S. Myers wrote:
> On Thu, 22 Mar 2012, Vincent Lefevre wrote:
> > For the same reason, if the user chose long double instead of
> > double, this may be because he wanted more precision than double.
> 
> You mean range?  IBM long double provides more precision, but not more 
> range.

Well, precision and/or range. If double precision format is sufficient
for his application, the user can just choose the "double" type. So,
I don't think that it is useful to have long double = double.

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).

> > 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.

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Vincent Lefèvre <vincent@vinc17.net> - Web: <http://www.vinc17.net/>
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