Record as argument to external function is not passed by reference

[c.schulze@…]

The Modelica Specification clearly states "Records are passed by reference (i.e. a pointer to the record is being passed)."

However, OpenModelica passes the record/struct instance instead of a pointer to the instance.

[Adrian Pop]

I thought this was an easy fix via PR:
​https://github.com/OpenModelica/OMCompiler/pull/2674
which fixed this model, *but* it doesn't seem so as
it breaks one model in our testsuite.
Spec says:

If there is a single output parameter and no explicit call of the external function, 
or if there is an explicit external call in the form of an equation, in which case the 
LHS must be one of the output parameters, the external routine is assumed to be a 
value-returning function. The element type T of an array can be any simple type as defined in Section 12.9.1.1 or, for C, a record type is returned as a value of the record type defined in Section 12.9.1.3.

So it seems we need more exceptions for the handling of the return from external functions.

If I get this right these are the cases:

// 1. no explicit call
// return structure, pass input by reference
// extern struct PLUS mk_plus_ext(struct ADD*, struct ADD*)
function mk_plus_ext
  input ADD left;
  input ADD right;
  output PLUS out;
  external "C" annotation(Library="blah");
end mk_plus_ext;

// 2. explicit call
// return structure, pass by reference
// extern struct PLUS mk_plus_ext(struct ADD*, struct ADD*)
function mk_plus_ext
  input ADD left;
  input ADD right;
  output PLUS out;
  external "C" out = ext(left, right) annotation(Library="blah");
end mk_plus_ext;

// 3. pass output by reference, explicitely
// pass by reference, including the output
// extern void mk_plus_ext(struct PLUS*, struct ADD*, struct ADD*)
function mk_plus_ext
  input ADD left;
  input ADD right;
  output PLUS out;
  external "C" ext(out, left, right) annotation(Library="blah");
end mk_plus_ext;

[Francesco Casella]

Replying to adrpo:

If I get this right these are the cases:

// 1. no explicit call
// return structure, pass input by reference
// extern struct PLUS mk_plus_ext(struct ADD*, struct ADD*)
function mk_plus_ext
  input ADD left;
  input ADD right;
  output PLUS out;
  external "C" annotation(Library="blah");
end mk_plus_ext;

// 2. explicit call
// return structure, pass by reference
// extern struct PLUS mk_plus_ext(struct ADD*, struct ADD*)
function mk_plus_ext
  input ADD left;
  input ADD right;
  output PLUS out;
  external "C" out = ext(left, right) annotation(Library="blah");
end mk_plus_ext;

// 3. pass output by reference, explicitely
// pass by reference, including the output
// extern void mk_plus_ext(struct PLUS*, struct ADD*, struct ADD*)
function mk_plus_ext
  input ADD left;
  input ADD right;
  output PLUS out;
  external "C" ext(out, left, right) annotation(Library="blah");
end mk_plus_ext;

I double-checked and it seems to me this is correct. The whole thing is horribly complicated, maybe it should have been made simpler, but it's too late to change that now, alas.

[Adrian Pop]

Fixed with 0cdd3e/OMCompiler.
More tests added with: 4b2bcb/OpenModelica-testsuite.