Where to attach backend builtin decl?

[Sa Liu]

> > The following is an example:
> >
> > module spu_intrinsics
> >   use, intrinsic :: iso_c_binding
> >   implicit none
> > contains
> >     function builtin_spu_add_int(a, b) result(c) bind(c,
> > name="__builtin_spu_add_1")
> >           integer (c_int), intent(in):: a(4), b(4)
> >           integer(c_int) :: c(4)
> >     end function builtin_vec_add_int
> > end module spu_intrinsics

> > In trans-decl.c function
> > gfc_get_extern_function_decl builds a new decl for the this function.
> 
> I would have said that you should interrupt the current codepath 
> before we get into gfc_get_extern_function_decl: check where that is 
> called, and when the symbol name starts with "__builtin_", instead of 
> calling gfc_get_extern_function_decl() call your own function that 
> looks up for the right intrinsic decl.

I looked at the code again and found out that 
gfc_get_extern_function_decl() is called, when a Fortran program calls the 
binding C function builtin_spu_add_int, which is handled as an external 
Fortran procedure. In this case the generated code is a brach to the 
backend intrinsic __builtin_spu_add_1. 

But the expansion of the builtin function should take place when resolving 
the module function builtin_spu_add_int. The function 
build_function_decl() is called at that time to generate a new decl for 
it, and the symbol it handles has name builtin_spu_add_int, with 
binding_label __builtin_spu_add_1. The decl I got from backend intrinsic, 
instead, has name __builtin_spu_add_1. In build_function_decl() I can not 
simply replace fndecl by the backend decl, since they have many different 
fields and attributes, and even names. I still have no idea where to 
insert the backend decl to expand the intrinsic function. Perhaps you know 
the internals code better...

Thanks!
Sa