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Subject: Re: [std-proposals] default floating point
From: Brian Bi (bbi5291_at_[hidden])
Date: 2021-01-14 20:59:12


I think you should articulate a clearer definition of "the default floating
point type". Does it mean the fastest floating-point type supported by the
architecture, regardless of precision? Or is the implementation allowed to
make a trade-off at their discretion, i.e., double provides X% more
precision than float, but is Y% faster, so comparing X and Y leads to the
decision of which type is the default?

Of course, the standard doesn't have such clear guidance for how long `int`
should be, but the length of `int` on each platform is a matter of
historical practice. With your proposed floating point type, such
historical practice doesn't exist yet so I think most users wouldn't want
to use it, given that they don't know what they're going to get.

A more flexible solution would be the float equivalent of the int_fastX_t
types, i.e., what is the fastest floating point type that provides at least
X bits of precision. Here is a sketch proposal to be added to
<numeric_limits>:

template <int precision, int exp_bits = 0>
using float_fast_t = *See below*;

float_fast_t<precision, exp_bits> is an alias for the fastest floating
point type supported by the implementation such that:

   - the smallest representable value after 1 is at most 1 + 2-precision,
   and
   - if exp_bits is greater than 0, then there exists a representable value
   that is at least 22exp_bits - 1.

If no such type exists, then any reference to the specialization
float_fast_t<precision,
exp_bits> is ill-formed.

On Thu, Jan 14, 2021 at 9:23 PM Vishal Oza via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> I was thinking of adding a default floating point type like int to the
> integer type rather than assume that double is the default. This might be
> better on older hardware where using a double might have a performance
> penalty. The keyword would either be flt or floating_point. I prefer flt
> for less typing but I can understand that it could break existing code. Can
> anyone arue why this is a bad idea?
>
> Vishal Oza
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>

-- 
*Brian Bi*


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