Re: mixed decimal floating and binary floating arithmetic

[Dan Zuras IEEE <forieee@xxxxxxxxxxxxxx>]

> From: "Fred J. Tydeman" <tydeman@xxxxxxxxx>
> To: "fred.zemke@xxxxxxxxxx" <fred.zemke@xxxxxxxxxx>,
>         "stds-754@xxxxxxxx" <stds-754@xxxxxxxx>
> Date: Wed, 15 Apr 2009 11:01:20 -0700 (PDT)
>
> On Wed, 15 Apr 2009 10:32:32 -0700, Fred Zemke wrote:
>
> > I don't see any guidance in Clause 10.1 "Expression evaluation rules"
> > about this situation, unless it is the last sentence "Language standards
> > should disallow, or provide warnings for, mixed format operations that
> > would cause implicit conversion that might change operand values". 
>
> The C language disallows (implicit) mixed format (binary FP and decimal FP) 
> operations in most cases.
>
> The C committee has issued a technical report on adding support for
> decimal floating point to the C language.  The relevant section from
> that TR is:
>
> ISO/IEC DTR 24732 WG14 N1312
>
> 6.4 Usual arithmetic conversions
>
> In an application that is written using decimal arithmetic, mixed
> operations between decimal and other real types are likely to occur
> only when interfacing with other languages, calling existing libraries
> written for binary floating point arithmetic, or accessing existing
> data. Determining the common type for mixed operations is difficult
> because ranges overlap; therefore, mixed mode operations are not
> allowed and the programmer must use explicit casts. Implicit conversions
> are allowed only for simple assignment, return statement, and in
> argument passing involving prototyped functions.
>
> ---
> Fred J. Tydeman        Tydeman Consulting


        Fred (Zemke),


        The statement in 10.1 IS the guidance.  That is, mixed radix
        operations should be disallowed.  It is a 'should' rather
        than a 'shall' because we didn't think we could prevent it
        in all cases.

        But if you search through the standard for the words 'mixed'
        or 'radix' you will find a number of statements that implicitly
        assume mixed radix operations are not an option.

        As Fred (Tydeman) shows, the C language committee has already
        followed this guidance for all the obvious reasons.

        It was our intent that Fortran disallow it as well.

        Realize that it is IMPLICIT conversions that are being
        disallowed.  You may still support mixed radix operations so
        long as the programmer specifies the radix of the operation
        by EXPLICITLY converting one operand to the other's radix.

        But it is a 'should' & you are free to decide this as you will.

        You know, I think this is the first time in my life I've had
        to further qualify my Freds in a statement. :-)

        Interesting...


                                   Dan