Where to attach backend builtin decl?

[Sa Liu]

I'm woking on the project to call backend builtin functions in Fortran. 
The backend builtins are C routines and I'll use ISO C binding to bind 
them with Fortran routines. 
The builtins are initialized by function gfc_builtin_function in 
f95-lang.c. Then the function pushdecl keeps the backend builtin decl in 
the current_binding_level. When resolving the function, where can I find 
these decls again?

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

The fortran function builtin_spu_add_int is bound with the backend builtin 
__builtin_spu_add_1. When resolving the symbol builtin_spu_add_int, the 
frontend can not find its decl. In trans-decl.c function 
gfc_get_extern_function_decl builds a new decl for the this function. I 
tried to set the fndecl to the builtins backend decl, but it seems not 
working. The question is where to attach the decl of the C bultin 
function, in order to generate inline code for the fortran program?

Thanks!
Sa