std.math performance (SSE vs. real)

[Walter Bright via Digitalmars-d]

On 7/2/2014 8:48 PM, Sean Kelly wrote:
> I'm still unclear whether we're aiming for C interoperability or hardware
> support though, based on Walter's remark about SPARC and PPC.  There, 'long
> double' is represented differently but is not backed by specialized hardware, so
> I'm guessing D would make 'real' 64-bits on these platforms and break
> compatibility with C.  So... I guess we really do need a special alias for C
> compatibility, and this can map to whatever intrinsic type the applicable
> compiler supports for that platform.

What is unclear about being able to call a C function declared as:

     int foo(long double r);

from D?