Translating Programs

The Lichen toolchain currently targets the C programming language, generating programs that are then compiled using existing C compilers. The process of translation involves the optimisation and generation of program structures, and the translation of Lichen language constructs into equivalent C language constructs.

Attribute Access Plans

During inspection, each attribute access is associated with a unique location and its details stored. During deduction, such accesses are resolved and characterised. During optimisation, such accesses are encoded as instruction plans indicating the operations or instructions required to achieve the access. During translation, these instruction plans are generated as part of the final program.

Structure Optimisation

All object structures need to support the attributes defined for them, and although it may be possible to determine the identity of attributes in advance of a program being run, there generally remains a need to provide mechanisms to determine the nature of an object at run-time and to access its attributes dynamically. To achieve this, an optimisation process catalogues the presence of each attribute name on all program objects, then deciding upon a position for the structure member corresponding to that attribute name in all objects.

Structure Generation

Each program consists of a number of structures providing the program's objects and defining classes, functions, modules and any predefined instances.

Attribute Representation

Attributes most typically provide references to objects within a certain optional context. Since attributes are employed to encode various other kinds of information, the data structure may be interpreted in other ways within suitably controlled environments. For example, the __data__ attribute found on instances of certain built-in classes will encode references to other kinds of information that are used within native functions.

Object Representation

Objects provide collections of attributes, and the object representation is meant to support efficient computed access to attributes whilst also allowing a reasonably efficient means of testing and accessing attributes at run-time.

Structure Constants

For clarity, several classes of symbolic constant are defined in the translated program to help define structures or to refer to structure members.

Constant Class

Purpose

Example

Application

csize

Indicates the number of attributes in a class

__csize___builtins___list_list

Structure definition

isize

Indicates the number of attributes in an instance

__isize___builtins___list_list

msize

Indicates the number of attributes in a module

__msize___builtins___list

pminsize

Indicates the minimum number of parameters expected by a callable

__pminsize___builtins___list_list_append

Invocation

pmaxsize

Indicates the maximum number of parameters expected by a callable

__pmaxsize___builtins___list_list_append

pcode

Assigns a code to a particular parameter name

__pcode_value

Keyword argument positioning

ppos

Indicates the table position used by a particular parameter name

__ppos_value

code

Assigns a code to a particular attribute name

__code_value

Attribute access

pos

Indicates the table position used by a particular attribute name

__pos_value

Such constants are encoded using helper functions, producing names resembling those above that are intended to be distinct and not to conflict with other defined names in the final program.

Instance Structures

Like all program objects, instances are defined in C according to a generic structure, meaning that all instances have the same generic members from the perspective of a C program. However, specific structures are defined for each class in order to conveniently describe the dimensions of instances of that class. For example, for the built-in list class, a declaration resembling the following is issued:

typedef struct {
    const __table * table;
    unsigned int pos;
    __attr attrs[__isize___builtins___list_list___init__];
} __obj___builtins___list_list___init__;

Program Translation

Useful C Language Features

The translated code relies on the availability of certain useful features of C, some of them supported only in modern compilers:

Function Naming

A naming scheme is applied to functions defined in the final C program to support different kinds of callables present in the original program.

Name Class

Purpose

Example

fn

Indicates a plain function

__fn___builtins___str_str

main

Indicates a module main program

__main_sys

new

Indicates a class instantiator

__new___builtins___list_list

newliteral

Provides an instantiator for literals mentioned in programs

__newliteral___builtins___list_list

Generic Program Functionality

Given the representation of the fundamental program objects, a suite of generic operations is provided to support the activities necessary to inspect and update program data. Such operations are largely concerned with generic object and attribute representations and are practically identical from program to program.

For example, an attribute provided by an object can be accessed in several different ways depending on the information known about the object when the program is translated. Consequently, a suite of attribute access operations is provided to support each different scenario:

Attribute provider

Class

Instance

Accessor

Class

load_via_object

Instance

load_via_class

load_via_object

Class or instance

get_class_and_load

check_and_load_via_any

Specific Program Functionality

Since objects at the program level have their layouts optimised, and since these layouts may differ from program to program, a suite of specific operations is provided to support a range of activities within programs, such operations being configured with program-specific information in order to do the right thing within a given program.

Function

Purpose

__invoke

A generic invocation function that populates the argument array and obtains the appropriate target

__new

Creates a new object, setting the __class__ attribute to the indicated class identity

__BOOL

Tests the identity of an attribute value against the True constant's address

__GETDEFAULT

Obtains a function default from additional members of a function instance beyond the core members

__SETDEFAULT

Updates a function default from additional members of a function instance beyond the core members

Module Files

Each program module is defined in its own file in the translated program. Since C has no notion of general module-level code, a special main function is generated for each module to perform the operations defined at the module level in the original program. These main functions are called in turn by the principal main function of the final C program, and the order of invocation is defined by the module initialisation ordering established when resolving inter-module dependencies.

Statement Translation

Statements in the original program are generally translated to statements in the final C program. During inspection and translation, certain constructs are transformed to produce the operations necessary to implement such constructs. For instance, operators need to be rewritten as function invocations, and thus the form of the original program changes before it is translated to C.

When translating certain operations that may appear within a statement, a sequence of instructions is generated, and such instructions must be performed in turn. Fortunately, C supports a wide variety of operations - such as assignments - within expressions, and it also provides the comma operator which permits sequences of operations to be performed in order and to yield a result that can then be used in the context of an expression. Such sequences of operations are employed extensively to realise attribute access instruction plans.

Name References

Names can be translated in different ways for the final C program. Function locals and parameters correspond directly to locals in C functions. Module-level globals and class-level locals are translated to structure accesses. However, temporary names that are used in sequence assignment are translated to C function locals, either in functions corresponding to their equivalents in the original program, or in module initialisation functions corresponding to the module scope in the original program.

Certain special names that are formulated during inspection are converted to constant or static references.

Attribute Accesses

Each attribute access is associated with a particular access location, corresponding to an operation occurring in the original program identified during inspection. This location can be used as a key to access a plan that describes how the access may be realised in the final program. Since most of the work computing the nature of the access is done during the preceding deduction and optimisation activities, what remains is mostly the emission of the plan into the generated program text with some substitutions performed.

Invocations

Each invocation involves an object whose identity may or may not be known at compile-time, plus a collection of arguments. The translation process is concerned with obtaining a target to be invoked in the generated program, populating an argument array appropriately for the target, and providing the means of invocation.

Where the identity of the object is not known at all, the information provided is effectively passed on to the special __invoke function which is then required to do all the work at run-time. However, one of the goals of analysing and compiling the program is to avoid doing such work at run-time and to take advantage of those cases where the identity of the object can be determined.

Thus, where the identity of the called object is known, details of the object's signature - its parameters and their positions - are used to populate the argument array and to determine whether this can be done successfully. Some callables require a context to be provided, and knowledge about the callable can be used to obtain or omit such a context from the arguments.

Finally, the invocation target must be obtained. Where some knowledge of the identity of the object is available, it may be sufficient to access the special __fn__ member directly (potentially testing for its presence if this is not certain) and to call the appropriate function with the assembled argument array. Where a definite identity is available, the actual function itself may be named and thus called with the arguments. Such operations are mere fragments of the total work performed by the __invoke function in the least optimal case.

Assignments

Assignments of objects to names occur for the following kinds of statements: assignments, definition statements (class and def), import statements (from and import).

Guards

Guards are identified during deduction as being necessary to uphold deductions about the nature of objects referenced via names that may be undermined at run-time by potentially erroneous program behaviour.

Exceptions

The C programming language does not support exceptions in a form supported by more modern programming languages. Consequently, lower-level mechanisms need to be employed to reproduce the semantics of the try statement, its except and finally clauses, and the raise statement, as well as considering the effect of exceptions on the return statement.

Fortunately, some consideration has been given to this topic before. The cexcept library - really a collection of convenience macros - provides a way of expressing exception constructs in a natural way in C while translating those constructs to usage of the setjmp and longjmp functions. Although direct usage of such functions could be generated during program translation, for convenience and to focus on other implementation concerns, it was decided to employ the cexcept macros and to build any additional functionality in terms of their usage.

A structure is defined to hold exception-related information:

typedef struct
{
    __attr arg;
    int raising;
    int raising_else;
    int completing;
} __exc;

Here, the arg member holds a raised exception object reference. Meanwhile, the remaining members interact with language constructs as follows:

Member

Description

Output Program Operation

raising

Indicates an exception that has been raised and not yet handled

__Raise

raising_else

Indicates an exception that must be re-raised because it occurred in an else clause

__RaiseElse

completing

Indicates the completion of, or return from, a try clause requiring entry into a finally clause

__Complete and __Return

Memory Allocation and Garbage Collection

To avoid having to write a garbage collector, the Boehm-Demers-Weiser garbage collector is employed by programs to allocate and free memory needed for its objects.