Translating functions into C, and the calling conventions needed for them.

• §1. Introduction
• §2. Predeclaration
• §4. Functions as values
• §5. Function calls
• §7. Declaring functions
• §12. Outward-bound function calls
• §13. Inward-bound function calls
• §15. Primitives for indirect or external function calls

§1. Introduction.

void CFunctionModel::initialise?Usage of CFunctionModel::initialise:
Final C - §1(code_generator *gtr) {
    METHOD_ADD(gtr, PREDECLARE_FUNCTION_MTID, CFunctionModel::predeclare_function);
    METHOD_ADD(gtr, DECLARE_FUNCTION_MTID, CFunctionModel::declare_function);
    METHOD_ADD(gtr, PLACE_LABEL_MTID, CFunctionModel::place_label);
    METHOD_ADD(gtr, EVALUATE_LABEL_MTID, CFunctionModel::evaluate_label);
    METHOD_ADD(gtr, INVOKE_FUNCTION_MTID, CFunctionModel::invoke_function);
}

typedef struct C_generation_function_model_data {
    int compiling_function;
    struct dictionary *predeclared_external_functions;
} C_generation_function_model_data;

void CFunctionModel::initialise_data?Usage of CFunctionModel::initialise_data:
Final C - §4(code_generation *gen) {
    C_GEN_DATA(fndata.compiling_function) = FALSE;
    C_GEN_DATA(fndata.predeclared_external_functions) = Dictionaries::new(1024, TRUE);
}

void CFunctionModel::begin?Usage of CFunctionModel::begin:
Final C - §5(code_generation *gen) {
    CFunctionModel::initialise_data(gen);
}

void CFunctionModel::end?Usage of CFunctionModel::end:
Final C - §6(code_generation *gen) {
    CFunctionModel::write_gen_call(gen);
    CFunctionModel::write_inward_calling_wrappers(gen);
}

• The structure C_generation_function_model_data is private to this section.

§2. Predeclaration. So, then: each Inter function will lead to a corresponding C function. The following places a predeclaration of that function high up in our C code, so that we then don't need to worry about code-ordering when compiling calls to it:

void CFunctionModel::predeclare_function?Usage of CFunctionModel::predeclare_function:
§1(code_generator *gtr, code_generation *gen,
    vanilla_function *vf) {
    segmentation_pos saved = CodeGen::select(gen, c_predeclarations_I7CGS);
    text_stream *OUT = CodeGen::current(gen);
    CFunctionModel::C_function_prototype(gen, OUT, vf);
    WRITE(";\n");
    CodeGen::deselect(gen, saved);
    CFunctionModel::declare_function_constant(gen, vf);
}

§3. And this expresses what the C prototype for that function will be. For example, suppose we have an Inter function which arises from a kit function reading like so:

    [ HelloThere greeting x y;
        ...
    ];

Now in practice this function may always be called as HelloThere("Aloha!") or similar, with the local variable greeting being an argument, and the other two locals x and y being used only internally. But Inter does not distinguish between arguments and private locals; and indeed it permits the same function to be called with differing numbers of arguments. HelloThere("Bonjour!", 31) is a legal call, and results in x initially being 31 rather than 0.

Because of this, our C analogue must also be callable with a variable number of arguments. Now, of course, C does have a crude mechanism for this (used for printf and similar), but it's nowhere near flexible enough to handle what we need. Instead we declare our C function like so:

    i7word_t i7_fn_HelloThere(i7process_t *proc, i7word_t i7_mgl_local_greeting,
        i7word_t i7_mgl_local_x, 7word_t i7_mgl_local_y) {
        ...
    }

And we then make calls like i7_fn_HelloThere(proc, X, 0, 0) or i7_fn_HelloThere(proc, X, Y, 0) to simulate calling this with one or two arguments respectively. Because unsupplied arguments are filled in as 0, we achieve the Inter convention that any local variables not used as call arguments are set to 0 at the start of a function. While this generates C code which does not look especially pretty, it works efficiently in practice.

We give the return type as i7word_t. In Inter, there is no such thing as a void function: all functions return something, even if that something is meaningless and is then thrown away.

void CFunctionModel::C_function_identifier?Usage of CFunctionModel::C_function_identifier:
§5, §6.1, §6.2, §14.1(code_generation *gen, OUTPUT_STREAM,
    vanilla_function *vf) {
    CNamespace::mangle_with(NULL, OUT, vf->identifier, I"fn");
}

void CFunctionModel::C_function_prototype?Usage of CFunctionModel::C_function_prototype:
§2, §8.1, §8.2(code_generation *gen, OUTPUT_STREAM,
    vanilla_function *vf) {
    WRITE("i7word_t ");
    CFunctionModel::C_function_identifier(gen, OUT, vf);
    WRITE("(i7process_t *proc");
    text_stream *local_name;
    LOOP_OVER_LINKED_LIST(local_name, text_stream, vf->locals) {
        WRITE(", i7word_t ");
        Generators::mangle(gen, OUT, local_name);
    }
    WRITE(")");
}

§4. Functions as values. A function is a value in Inter, and can be stored in variables, or arrays, or properties, and so on. The Vanilla algorithm expects that the mangled name of the function identifier will evaluate to this value.

But what is this value to be? The obvious thing would be to represent the Inter function at runtime by a pointer to the C function it has become. We could then dereference that pointer to perform a function call, and so on. But this doesn't work, because C does not allow function pointers to be used in a constant context.

This is why the function CFunctionModel::C_function_identifier writes an identifier which is _not_ the same as the mangled function name. Instead, the mangled function name is defined as a constant, as follows.

void CFunctionModel::declare_function_constant?Usage of CFunctionModel::declare_function_constant:
§2(code_generation *gen, vanilla_function *vf) {
    segmentation_pos saved = CodeGen::select(gen, c_function_predeclarations_I7CGS);
    text_stream *OUT = CodeGen::current(gen);
    WRITE("#define ");
    CNamespace::mangle(NULL, OUT, vf->identifier);
    WRITE(" (I7VAL_FUNCTIONS_BASE + %d)\n", vf->allocation_id);
    CodeGen::deselect(gen, saved);
}

§5. Function calls. First, the straightforward and most common case: calling a function whose identity is known at run-time, and is passed to us as vf.

void CFunctionModel::invoke_function?Usage of CFunctionModel::invoke_function:
§1(code_generator *gtr, code_generation *gen,
    inter_tree_node *P, vanilla_function *vf, int void_context) {
    text_stream *OUT = CodeGen::current(gen);
    CFunctionModel::C_function_identifier(gen, OUT, vf);
    WRITE("(proc");
    if (vf->takes_variable_arguments) Supply arguments on the stack5.2
    else Supply arguments as call parameters5.1;
    WRITE(")");
    if (void_context) WRITE(";\n");
}

§5.1. As noted above, all unsupplied call parameters are filled in with zeroes:

Supply arguments as call parameters5.1 =

    int c = 0;
    LOOP_THROUGH_INTER_CHILDREN(F, P) {
        WRITE(", "); Vanilla::node(gen, F);
        c++;
    }
    for (; c < vf->max_arity; c++) WRITE(", 0");

• This code is used in §5.

§5.2. A handful of functions, always supplied by kits (i.e., never compiled directly by Inform 7), use a stack-based calling mechanism instead. For example:

[ whatever _vararg_count ret;
    ...
];

The significant thing here is that the first local variable is called _vararg_count. The Inform 6 compiler reacts to that by using a different function call mechanism; the function call whatever(x, y, z) will then result in x, y, z being pushed onto the stack, while execution in the function begins with _vararg_count equal to 3 (the number of things pushed), and ret equal to 0.

Note that the arguments must be pushed in reverse order — z, y, x — in order to ensure that the first one, x, is at the top of the stack when execution of the function begins.

But of course a C compiler will not automatically do that just because the first local happens to be called _vararg_count, so we must simulate the effect here.

In practice, the maximum number of variable arguments needed is seldom more than about 3 and never more than 10 in I7 usage, so the maximum here is not at all restrictive.

define MAX_VARARG_COUNT 128

Supply arguments on the stack5.2 =

    inter_tree_node *args[MAX_VARARG_COUNT];
    int c = 0;
    LOOP_THROUGH_INTER_CHILDREN(F, P) if (c < MAX_VARARG_COUNT) args[c++] = F;
    WRITE(", (");
    for (int i=c-1; i >= 0; i--) {
        WRITE("i7_push(proc, ");
        Vanilla::node(gen, args[i]);
        WRITE("), ");
    }
    WRITE("%d)", c);
    for (int i=1; i < vf->max_arity; i++) WRITE(", 0");

• This code is used in §5.

§6. Now the harder case: calling a function whose identity is not known at compile time, but which is identified only as a runtime value which will be one of the numbers defined above. This is done with a function called i7_gen_call, and that is what we must now compile:

i7word_t i7_gen_call(i7process_t *proc, i7word_t id, i7word_t *args, int argc);

• This is part of the extract file inform7_clib.h.

The structure is basically one big switch. In simplified pseudocode it looks like so:

    switch (function_id_number) {
        case id1: rv = fn1(proc, arg1); break;
        case id2: rv = fn2(proc, arg1, arg2, arg3); break;
        ...
    }

That seems inefficient: instead, why not store the addresses of the relevant functions in a lookup table? After all, the case ID numbers are just a consecutive run of integers.

But we can't do that because in the C standard there is no safe way to cast or store those pointer types in a way which would safely be a union of the possible function types. All of the things which probably work on most architectures are formally "undefined behavior" in C99; we cannot, for example, assume that function pointers have the same size (in the sizeof sense) as other pointers, or even that a pointer to a function of three arguments has the same size as a pointer to a function of two. (Almost certainly it does: but the C99 standard is pretty clear that you take your life into your own hands making these casual assumptions.)

On the brighter side, though, modern C compilers are good at compiling switch statements with easy-to-index case numbers in an efficient way: so what we cannot legally express in source code will quite likely be what it compiles anyway, and it is unlikely that i7_gen_call will be slow.

void CFunctionModel::write_gen_call?Usage of CFunctionModel::write_gen_call:
§1(code_generation *gen) {
    segmentation_pos saved = CodeGen::select(gen, c_function_callers_I7CGS);
    text_stream *OUT = CodeGen::current(gen);
    WRITE("i7word_t i7_gen_call(\n");
    WRITE("i7process_t *proc, i7word_t id, i7word_t *args, int argc) {\n"); INDENT;
    WRITE("i7word_t rv = 0;\n");
    WRITE("switch (id) {\n"); INDENT;
    WRITE("case 0: rv = 0; break;\n");
    vanilla_function *vf;
    LOOP_OVER(vf, vanilla_function) {
        WRITE("case ");
        CNamespace::mangle(NULL, OUT, vf->identifier);
        WRITE(": ");
        if (vf->takes_variable_arguments) Supply general arguments on the stack6.2
        else Supply general arguments as call parameters6.1;
        WRITE(" break;\n");
    }
    WRITE("default: printf(\"function %%d not found\\n\", id); i7_fatal_exit(proc); break;\n");
    OUTDENT; WRITE("}\n");
    WRITE("return rv;\n");
    OUTDENT; WRITE("}\n");
    CodeGen::deselect(gen, saved);
}

§6.1. Supply general arguments as call parameters6.1 =

    WRITE("rv = ");
    CFunctionModel::C_function_identifier(gen, OUT, vf);
    WRITE("(proc");
    for (int i=0; i<vf->max_arity; i++) WRITE(", args[%d]", i);
    WRITE(");");

• This code is used in §6.

§6.2. Supply general arguments on the stack6.2 =

    WRITE("for (int i=argc-1; i>=0; i--) i7_push(proc, args[i]); ");
    WRITE("rv = ");
    CFunctionModel::C_function_identifier(gen, OUT, vf);
    WRITE("(proc, argc");
    for (int i=1; i<vf->max_arity; i++) WRITE(", 0");
    WRITE(");\n");

• This code is used in §6.

§7. Declaring functions. This is now straightforward except for one last annoying part of the Inter calling convention.

It is legal for an Inter function to leave pushed values still on the stack when it exits. If the function takes varargs, those values allow for exotic forms of return value, and are left there for the caller to deal with (or not). But if it's a regular, non-varargs sort of function, then the stack must be restored to the status quo ante when the function returns, just as if it had properly pulled all the values it had pushed.

We deal with this by having a "stack-safe" version of the function whose only job is to save the stack pointer, call the "unsafe" (i.e. real) version of the function, then restore the stack pointer again.

The C types of the safe and unsafe functions are identical, so the following look much like CFunctionModel::C_function_identifier and CFunctionModel::C_function_prototype.

void CFunctionModel::unsafe_C_function_identifier?Usage of CFunctionModel::unsafe_C_function_identifier:
§8.2(code_generation *gen, OUTPUT_STREAM,
    vanilla_function *vf) {
    CNamespace::mangle_with(NULL, OUT, vf->identifier, I"ifn");
}

void CFunctionModel::unsafe_C_function_prototype?Usage of CFunctionModel::unsafe_C_function_prototype:
§8.1(code_generation *gen, OUTPUT_STREAM,
    vanilla_function *vf) {
    WRITE("i7word_t ");
    CFunctionModel::unsafe_C_function_identifier(gen, OUT, vf);
    WRITE("(i7process_t *proc");
    text_stream *local_name;
    LOOP_OVER_LINKED_LIST(local_name, text_stream, vf->locals) {
        WRITE(", i7word_t ");
        Generators::mangle(gen, OUT, local_name);
    }
    WRITE(")");
}

§8.

void CFunctionModel::declare_function?Usage of CFunctionModel::declare_function:
§1(code_generator *gtr, code_generation *gen,
    vanilla_function *vf) {
    text_stream *fn_name = vf->identifier;
    segmentation_pos saved = CodeGen::select(gen, c_function_declarations_I7CGS);
    text_stream *OUT = CodeGen::current(gen);
    Compile the functional part8.1;
    if (vf->takes_variable_arguments == FALSE) Compile the stack-safe outer function8.2;
    CodeGen::deselect(gen, saved);
}

§8.1. Compile the functional part8.1 =

    if (vf->takes_variable_arguments) CFunctionModel::C_function_prototype(gen, OUT, vf);
    else CFunctionModel::unsafe_C_function_prototype(gen, OUT, vf);
    WRITE(" {\n");
    WRITE("i7_debug_stack(\"%S\");\n", fn_name);
    if (Str::eq(fn_name, I"DebugAction")) {
        WRITE("switch (i7_mgl_local_a) {\n");
        text_stream *aname;
        LOOP_OVER_LINKED_LIST(aname, text_stream, gen->actions) {
            WRITE("case i7_ss_%S", aname);
            WRITE(": printf(\"%S\"); return 1;\n", aname);
        }
        WRITE("}\n");
    }
    C_GEN_DATA(fndata.compiling_function) = TRUE;
    Vanilla::node(gen, vf->function_body);
    C_GEN_DATA(fndata.compiling_function) = FALSE;
    WRITE("return 1;\n");
    WRITE("\n}\n\n");

• This code is used in §8.

§8.2. Compile the stack-safe outer function8.2 =

    CFunctionModel::C_function_prototype(gen, OUT, vf);
    WRITE(" {\n");
    WRITE("int ssp = proc->state.stack_pointer;\n");
    WRITE("i7word_t rv = ");
    CFunctionModel::unsafe_C_function_identifier(gen, OUT, vf);
    WRITE("(proc");
    text_stream *local_name;
    LOOP_OVER_LINKED_LIST(local_name, text_stream, vf->locals) {
        WRITE(", ");
        Generators::mangle(gen, OUT, local_name);
    }
    WRITE(");\n");
    WRITE("proc->state.stack_pointer = ssp;\n", vf->identifier);
    WRITE("return rv;\n");
    WRITE("\n}\n\n");

• This code is used in §8.

§9.

int CFunctionModel::inside_function?Usage of CFunctionModel::inside_function:
C Arithmetic - §1(code_generation *gen) {
    if (C_GEN_DATA(fndata.compiling_function)) return TRUE;
    return FALSE;
}

§10. Labels can be placed in C code with the notation LabelName:, but note that in C it is a syntax error for a label to occur at the end of a block, e.g., while (1) { ...; EndOfLoop: } is a syntax error. This can be put right with an empty statement, i.e., a semicolon: while (1) { ...; EndOfLoop: ; } And in case that is what we need here, we always place an empty statement after a label.

void CFunctionModel::place_label?Usage of CFunctionModel::place_label:
§1(code_generator *gtr, code_generation *gen,
    text_stream *label_name) {
    text_stream *OUT = CodeGen::current(gen);
    LOOP_THROUGH_TEXT(pos, label_name)
        if (Str::get(pos) != '.')
            PUT(Str::get(pos));
    WRITE(": ;\n", label_name);
}

§11. Labels are not really "evaluated" in C: goto destinations are not values. Evaluation in this sense just means compiling the name used a sort of argument to the goto statement.

Note that label names, whose scope is confined to the function in which they occur, are unmangled. This is safe because label names have their own namespace in C, so they cannot clash with other identifiers.

void CFunctionModel::evaluate_label?Usage of CFunctionModel::evaluate_label:
§1(code_generator *gtr, code_generation *gen,
    text_stream *label_name) {
    text_stream *OUT = CodeGen::current(gen);
    LOOP_THROUGH_TEXT(pos, label_name)
        if (Str::get(pos) != '.')
            PUT(Str::get(pos));
}

§12. Outward-bound function calls. "Outward" here means "when a function compiled from Inter makes a call to a C function from the world outside, i.e., which wasn't compiled from Inter". This is done with the !externalcall primitive: see below.

In order to make the linking work, we need to ensure that our code declares the external function's name before use. But we should do this only for those functions we actually need to call, and only once for each of them. So we keep a dictionary of those already declared.

Note that all external identifiers begin with external__, which is 10 characters long.

text_stream *CFunctionModel::ensure_external_function_predeclared?Usage of CFunctionModel::ensure_external_function_predeclared:
§15.1(code_generation *gen,
    text_stream *external_identifier) {
    dictionary *D = C_GEN_DATA(fndata.predeclared_external_functions);
    text_stream *key = Str::new();
    for (int i=10; i<Str::len(external_identifier); i++)
        PUT_TO(key, Str::get_at(external_identifier, i));
    text_stream *dv = Dictionaries::get_text(D, key);
    if (dv == NULL) {
        Dictionaries::create_text(D, key);
        segmentation_pos saved = CodeGen::select(gen, c_predeclarations_I7CGS);
        text_stream *OUT = CodeGen::current(gen);
        WRITE_TO(OUT, "i7word_t %S(i7process_t *proc, i7word_t arg);\n", key);
        CodeGen::deselect(gen, saved);
    }
    return key;
}

§13. Inward-bound function calls. "Inward" here means "when a C function not compiled from Inter calls one which is": that is, when the world outside wants to call into some portion of an Inform program.

In principle, the caller could just imitate our own calling convention, i.e., could do something like what CFunctionModel::invoke_function does. But in practice this is a messy business, in that it makes for illegible code, so we provide something easier on the eyes.

In particular, for each function arising from Inform 7 source text, we deduce the "formal arity" (the actual number of arguments it takes) and make a wrapper function which has just those as arguments. The wrapper then calls the real function, filling in all the bogus arguments as 0.

void CFunctionModel::inward_C_function_identifier?Usage of CFunctionModel::inward_C_function_identifier:
§14.3(code_generation *gen, OUTPUT_STREAM,
    vanilla_function *vf) {
    CNamespace::mangle_with(NULL, OUT, vf->identifier, I"xfn");
}

void CFunctionModel::inward_C_function_prototype?Usage of CFunctionModel::inward_C_function_prototype:
§14.1, §14.2(code_generation *gen, OUTPUT_STREAM,
    vanilla_function *vf) {
    WRITE("i7word_t ");
    CFunctionModel::inward_C_function_identifier(gen, OUT, vf);
    WRITE("(i7process_t *proc");
    for (int i=0; i<vf->formal_arity; i++) WRITE(", i7word_t p%d", i);
    WRITE(")");
}

§14.

void CFunctionModel::write_inward_calling_wrappers?Usage of CFunctionModel::write_inward_calling_wrappers:
§1(code_generation *gen) {
    vanilla_function *vf;
    LOOP_OVER(vf, vanilla_function) {
        Compile a wrapper function for inward calling14.1;
        Define a more friendly alias for the wrapper function name14.3;
        Compile a predeclaration for the wrapper function for inward calling14.2;
    }
}

§14.1. This goes into the main C file we are compiling.

Compile a wrapper function for inward calling14.1 =

    segmentation_pos saved = CodeGen::select(gen, c_function_callers_I7CGS);
    text_stream *OUT = CodeGen::current(gen);
    CFunctionModel::inward_C_function_prototype(gen, OUT, vf); WRITE(" {\n"); INDENT;
    WRITE("return ");
    CFunctionModel::C_function_identifier(gen, OUT, vf);
    WRITE("(proc");
    for (int i=0; i<vf->max_arity; i++) {
        WRITE(", ");
        if (i < vf->formal_arity) WRITE("p%d", i); else WRITE("0");
    }
    WRITE(");\n");
    OUTDENT; WRITE("}\n");
    CodeGen::deselect(gen, saved);

• This code is used in §14.

§14.2. This goes into the secondary header file, and predeclares the wrapper function for linking purposes.

Compile a predeclaration for the wrapper function for inward calling14.2 =

    segmentation_pos saved = CodeGen::select(gen, c_function_symbols_I7CGS);
    text_stream *OUT = CodeGen::current(gen);
    CFunctionModel::inward_C_function_prototype(gen, OUT, vf); WRITE(";\n");
    CodeGen::deselect(gen, saved);

• This code is used in §14.

§14.3. Similarly, this for the header file produces a nicer name which can be used for the wrapper.

Define a more friendly alias for the wrapper function name14.3 =

    TEMPORARY_TEXT(synopsis)
    VanillaFunctions::syntax_synopsis(synopsis, vf);
    TEMPORARY_TEXT(val)
    CFunctionModel::inward_C_function_identifier(gen, val, vf);
    segmentation_pos saved = CodeGen::select(gen, c_function_symbols_I7CGS);
    text_stream *OUT = CodeGen::current(gen);
    WRITE("#define %S %S\n", CTarget::symbols_header_identifier(gen, I"F", synopsis), val);
    CodeGen::deselect(gen, saved);
    DISCARD_TEXT(val)
    DISCARD_TEXT(synopsis)

• This code is used in §14.

§15. Primitives for indirect or external function calls. Most function calls are made explicitly: see CFunctionModel::invoke_function above. But the Inter primitives below offer a way to call functions whose identities are not known at compile time, or which are not even part of the Inter program.

The following primitives all simply call functions i7_call_0, and so on — see below for their definitions — except for !externalcall.

int CFunctionModel::invoke_primitive?Usage of CFunctionModel::invoke_primitive:
C Program Control - §1(code_generation *gen, inter_ti bip, inter_tree_node *P) {
    text_stream *OUT = CodeGen::current(gen);
    switch (bip) {
        case INDIRECT0_BIP: case INDIRECT0V_BIP:
            WRITE("i7_call_0(proc, "); VNODE_1C; WRITE(")"); break;
        case INDIRECT1_BIP: case INDIRECT1V_BIP:
            WRITE("i7_call_1(proc, "); VNODE_1C; WRITE(", ");
            VNODE_2C; WRITE(")"); break;
        case INDIRECT2_BIP: case INDIRECT2V_BIP:
            WRITE("i7_call_2(proc, "); VNODE_1C; WRITE(", ");
            VNODE_2C; WRITE(", "); VNODE_3C; WRITE(")"); break;
        case INDIRECT3_BIP: case INDIRECT3V_BIP:
            WRITE("i7_call_3(proc, "); VNODE_1C; WRITE(", ");
            VNODE_2C; WRITE(", "); VNODE_3C; WRITE(", "); VNODE_4C; WRITE(")"); break;
        case INDIRECT4_BIP: case INDIRECT4V_BIP:
            WRITE("i7_call_4(proc, "); VNODE_1C; WRITE(", ");
            VNODE_2C; WRITE(", "); VNODE_3C; WRITE(", "); VNODE_4C; WRITE(", ");
            VNODE_5C; WRITE(")"); break;
        case INDIRECT5_BIP: case INDIRECT5V_BIP:
            WRITE("i7_call_5(proc, "); VNODE_1C; WRITE(", ");
            VNODE_2C; WRITE(", "); VNODE_3C; WRITE(", "); VNODE_4C; WRITE(", ");
            VNODE_5C; WRITE(", "); VNODE_6C; WRITE(")"); break;
        case MESSAGE0_BIP:
            WRITE("i7_mcall_0(proc, "); VNODE_1C; WRITE(", "); VNODE_2C; WRITE(")"); break;
        case MESSAGE1_BIP:
            WRITE("i7_mcall_1(proc, "); VNODE_1C; WRITE(", "); VNODE_2C; WRITE(", ");
            VNODE_3C; WRITE(")"); break;
        case MESSAGE2_BIP:
            WRITE("i7_mcall_2(proc, "); VNODE_1C; WRITE(", "); VNODE_2C; WRITE(", ");
            VNODE_3C; WRITE(", "); VNODE_4C; WRITE(")"); break;
        case MESSAGE3_BIP:
            WRITE("i7_mcall_3(proc, "); VNODE_1C; WRITE(", "); VNODE_2C; WRITE(", ");
            VNODE_3C; WRITE(", "); VNODE_4C; WRITE(", "); VNODE_5C; WRITE(")"); break;
        case EXTERNALCALL_BIP:
            Generate primitive for externalcall15.1; break;
        default:
            return NOT_APPLICABLE;
    }
    return FALSE;
}

§15.1. Generate primitive for externalcall15.1 =

    inter_tree_node *N = InterTree::first_child(P);
    if (Inode::is(N, VAL_IST)) {
        inter_pair val = ValInstruction::value(N);
        if (InterValuePairs::is_text(val)) {
            text_stream *text = InterValuePairs::to_text(gen->from, val);
            WRITE("%S(proc, ",
                CFunctionModel::ensure_external_function_predeclared(gen, text));
            VNODE_2C; WRITE(")");
        } else {
            internal_error("unimplemented form of !externalcall");
        }
    } else {
        internal_error("unimplemented form of !externalcall");
    }

• This code is used in §15.

§16. The following functions implement the above. i7_call_N provides a general way to call an Inter function with N arguments, up to 5.

i7word_t i7_call_0(i7process_t *proc, i7word_t id);
i7word_t i7_call_1(i7process_t *proc, i7word_t id, i7word_t v);
i7word_t i7_call_2(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2);
i7word_t i7_call_3(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2, i7word_t v3);
i7word_t i7_call_4(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2, i7word_t v3,
    i7word_t v4);
i7word_t i7_call_5(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2, i7word_t v3,
    i7word_t v4, i7word_t v5);

• This is part of the extract file inform7_clib.h.

But these are not really different from each other: they all simply call i7_gen_call, the function we compiled laboriously in CFunctionModel::write_gen_call above, to do the actual business.

i7word_t i7_call_0(i7process_t *proc, i7word_t id) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    return i7_gen_call(proc, id, args, 0);
}
i7word_t i7_call_1(i7process_t *proc, i7word_t id, i7word_t v) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v;
    return i7_gen_call(proc, id, args, 1);
}
i7word_t i7_call_2(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v; args[1] = v2;
    return i7_gen_call(proc, id, args, 2);
}
i7word_t i7_call_3(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2,
    i7word_t v3) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v; args[1] = v2; args[2] = v3;
    return i7_gen_call(proc, id, args, 3);
}
i7word_t i7_call_4(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2,
    i7word_t v3, i7word_t v4) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v; args[1] = v2; args[2] = v3; args[3] = v4;
    return i7_gen_call(proc, id, args, 4);
}
i7word_t i7_call_5(i7process_t *proc, i7word_t id, i7word_t v, i7word_t v2,
    i7word_t v3, i7word_t v4, i7word_t v5) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v; args[1] = v2; args[2] = v3; args[3] = v4; args[4] = v5;
    return i7_gen_call(proc, id, args, 5);
}

• This is part of the extract file inform7_clib.c.

§17. The following functions implement the above. i7_mcall_N provides a general way to make a "message call" to an Inter function with N arguments, up to 3. Message calls are really the same as regular function calls, except that the function ID is read from a property of an object, and except that the self variable has to be set to that object when the function is running (and restored back to its previous value afterwards). Again, we use i7_gen_call to do the actual work.

i7word_t i7_mcall_0(i7process_t *proc, i7word_t to, i7word_t prop);
i7word_t i7_mcall_1(i7process_t *proc, i7word_t to, i7word_t prop, i7word_t v);
i7word_t i7_mcall_2(i7process_t *proc, i7word_t to, i7word_t prop, i7word_t v,
    i7word_t v2);
i7word_t i7_mcall_3(i7process_t *proc, i7word_t to, i7word_t prop, i7word_t v,
    i7word_t v2, i7word_t v3);

• This is part of the extract file inform7_clib.h.

i7word_t i7_mcall_0(i7process_t *proc, i7word_t to, i7word_t prop) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    i7word_t saved = proc->state.variables[i7_var_self];
    proc->state.variables[i7_var_self] = to;
    i7word_t id = i7_read_prop_value(proc, to, prop);
    i7word_t rv = i7_gen_call(proc, id, args, 0);
    proc->state.variables[i7_var_self] = saved;
    return rv;
}

i7word_t i7_mcall_1(i7process_t *proc, i7word_t to, i7word_t prop, i7word_t v) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v;
    i7word_t saved = proc->state.variables[i7_var_self];
    proc->state.variables[i7_var_self] = to;
    i7word_t id = i7_read_prop_value(proc, to, prop);
    i7word_t rv = i7_gen_call(proc, id, args, 1);
    proc->state.variables[i7_var_self] = saved;
    return rv;
}

i7word_t i7_mcall_2(i7process_t *proc, i7word_t to, i7word_t prop, i7word_t v,
    i7word_t v2) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v; args[1] = v2;
    i7word_t saved = proc->state.variables[i7_var_self];
    proc->state.variables[i7_var_self] = to;
    i7word_t id = i7_read_prop_value(proc, to, prop);
    i7word_t rv = i7_gen_call(proc, id, args, 2);
    proc->state.variables[i7_var_self] = saved;
    return rv;
}

i7word_t i7_mcall_3(i7process_t *proc, i7word_t to, i7word_t prop, i7word_t v,
    i7word_t v2, i7word_t v3) {
    i7word_t args[10]; for (int i=0; i<10; i++) args[i] = 0;
    args[0] = v; args[1] = v2; args[2] = v3;
    i7word_t saved = proc->state.variables[i7_var_self];
    proc->state.variables[i7_var_self] = to;
    i7word_t id = i7_read_prop_value(proc, to, prop);
    i7word_t rv = i7_gen_call(proc, id, args, 3);
    proc->state.variables[i7_var_self] = saved;
    return rv;
}

• This is part of the extract file inform7_clib.c.