[Cfp-interest] implementing tgmath for constant rounding modes

[Joel C. Salomon]

On 06/07/2012 03:48 PM, Fred J. Tydeman wrote:
>
> On Sat, 2 Jun 2012 08:22:55 -0700 Jim Thomas wrote:
>>
>> <tgmath.h> aside, the math functions that can be affected by constant rounding modes must be 
>> sensitive to whether a constant rounding mode is in effect. 
>> Take the example of cbrt. <math.h> could have
>>
>> #define cbrt(x) __roundwise_cbrt(x)
>>
>> Then the translator would have to keep track of whether a constant rounding mode is in effect at 
>> each call site for __roundwise_cbrt and, if so, what the rounding direction is. 
>>
>> Compilers already keep track of the state of other FP pragmas, which have the same sort of 
>> scoping as the constant rounding mode pragmas.
> 
> As far as I recall, neither gcc nor clang keep track of FENV_ACCESS.  So, I am not
> sure how easy it is to track state of FP pragmas.
> 
> So, in effect, __roundwise_cbrt becomes a special keyword/token/symbol to the compiler.
> And the same for all the other math functions (that is a lot of names to treat special).
> 
> So, if another compiler did it as
>   #define cbrt(x) __RoundWise_cbrt(x)
> it will be hard for 3rd party library/header implementors
> (such as P.J.Plauger's Dinkumware) to create a header that 
> works with all compilers.
> 
> Would it be better to add just one new attribute, eg,
>   #define cbrt(x) [[ROUNDWISE]] cbrt(x)
> that would be the same for all compilers?

Except that attributes were not accepted into C11.

Also, looking at <http://open-std.org/jtc1/sc22/wg14/www/docs/n1403.pdf>
where attributes for C++ are described, it does not seem to me that they
can be used in this way, only for declarations and listed control
constructs.

So you'd be left with including [[roundwise]] in the function
declaration, e.g.:

    double cbrt [[roundwise]] (double);

(which cannot be evaded by macro suppression), or with a cast, e.g.:

    typedef double __roundwise_dbl [[roundwise]] (double);
    #define cbrt(x) ((__roundwise)cbrt)(x)

but does anyone know if such a cast does what I'm trying for in C++?

--Joel