Why is it that no one writes with portability in mind in druntime?

[Iain Buclaw]

On 25 November 2013 21:44, IgorStepanov <wazar at mail.ru> wrote:
> On Sunday, 24 November 2013 at 11:38:27 UTC, Iain Buclaw wrote:
>>
>> https://github.com/D-Programming-Language/druntime/pull/663
>
>
> This code is not portable only on CTFE part. (All those float manipulations
> run at compile time)
> I see three unportable aspects:
> 1. This code supports only 80bit real. What other formats we need to
> support? Can  the compiler support operations on those (or more precise)
> formats? AFAIK dmd frontend perform all float operations in 80 bit real and
> supports only x86.

The following IEEE 'real' formats are currently supported:
64 bit Big-endian  'double' (eg PowerPC)
128 bit Big-endian 'quadruple' (eg SPARC)
64 bit Little-endian 'double' (eg x86-SSE2)
80 bit Little-endian, with implied bit 'real80' (eg x87, Itanium).
128 bit Little-endian 'quadruple' (not implemented on any known processor!)

Non-IEEE 128 bit Big-endian 'doubledouble' (eg PowerPC) has partial support

At least support for these is a must:

version(LittleEndian)
{
    static assert(real.mant_dig == 53 || real.mant_dig==64
               || real.mant_dig == 113);
}
else
{
    static assert(real.mant_dig == 53 || real.mant_dig == 113);
}

128 bit doubledouble formats (real.mant_dig == 106) - are a future goal.


> 2. This code don't considers a float byte order. But compiler don't inform
> user code about float BO. There are LittleEndian/BigEndian versions for
> integers, but not for floats. You can add this versions on compiler and I'll
> fix this aspect in druntime code.

There doesn't exist a machine where the most significant byte of a
word is ordered differently to floats. See std.math if in doubt, as
this is already implemented.

> 3. FloatTraits. Yes, thay works only with x86. Please, get link to other
> supported float formats and I'll extend FloatTraits to those formats.
>

See std.math, which already has a cross-platform implementation of
that template with the same name (that amazingly existed years ago).