Again: FFI syntax

[Fergus Henderson]

On 29-May-2001, Manuel M. T. Chakravarty <chak at cse.unsw.edu.au> wrote:
> Sven Panne <Sven_Panne at BetaResearch.de> wrote,
> 
> > Fergus Henderson wrote:
> > 
> > > The calling convention should not necessarily default to 'ccall'.
> > > That would not be appropriate for all implementations.
> > 
> > Granted.
> > 
> > > Instead, I think the default calling convention should be
> > > implementation-dependent.
> > 
> > Hmmm, this would make the semantics of the sources compiler-/interpreter-
> > dependent, which is never a good thing. 

Making the semantics of a particular construct implementation-dependent is
a good thing if the semantics that you want are implementation-dependent.
Doing this allows the code to work correctly on different implementations
without modifying the code.

That is the case here, I believe.  In particular, "ccall" is not abstract
enough to use as a default.  The default calling convention -- the one that
you normally want -- is "whatever the corresponding C implementation uses".
But on x86, "ccall" means a *particular* calling convention (args passed
on stack, caller pops, return value in EAX, etc.) which may not match
the one that your C implementation uses.  If you have a C implementation
which always passes args in registers, as is certainly allowed by the C
standard, then you want to the default calling convention to be passing
args in registers, otherwise nothing that uses the default will work.

> > > Also, implementations should be allowed to provide calling conventions
> > > not on that list.
> > 
> > IIRC the intention for enumerating some calling conventions here is to
> > make future implementations agree on the name of callconv when they
> > implement e.g. a C++ or Java FFI, not to rule out other possibilities.
> 
> Exactly.
> 
> > But this should be made clear from the FFI spec.
> 
> The spec said,
> 
>   Generally, the set of calling conventions is open, as it
>   is infeasible to cover all useful calling conventions in
>   this report.
> 
> I have changed this now to
> 
>   Generally, the set of calling conventions is open, i.e.,
>   individual implementations may elect to support additional
>   calling conventions.

The words are clear, but IIRC they contradict the grammar.
It would be nicer to make this explicit in the grammar.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
                                    |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.