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MysoreScript

MysoreScript is a simple language designed to demonstrate the important aspects of JavaScript for compilation. It has classes, closures and objects, but only integer arithmetic.

MysoreScript is not intended to be used seriously, it is a toy language that embodies the aspects of JavaScript that make it difficult to compile efficiently, but lacks most of the complexity.

The implementation provides the ability to load code from a file and to run an interactive environment.

Operators

MysoreScript supports four arithmetic operators, which are equivalent to invoking the following methods:

When used on values of the Number type, they have their conventional meanings.

MysoreScript also supports the C binary comparison operators. Equality on non-number values is defined as object identity. Ordered comparisons have undefined behavior.

Primitive Object Types

MysoreScript includes several primitive object types. Number is used for all number literals and implements a small number of methods. Array is used to represent variable-sized ordered collections of data. The String type contains immutable strings and provides character-level accesses.

Unlike Smalltalk, classes are not first class objects. MysoreScript also lacks class methods.

Closure Binding

In MysoreScript, values are bound to closures when they are created. For example, consider the following simple program:

func outer()
{
	var str = "old value\n";
	func inner()
	{
		str.dump();
	};
	str = "new value\n";
	return inner;
};
outer()();

This will print ‘old value’, not ‘new value’.

Simplifications

MysoreScript is intended to capture enough of the difficulties in compilation exposed by JavaScript to be useful in teaching, but not so many of them that it is completely horrible to work with.

There are several things that make MysoreScript easier to compile than JavaScript. The first is that it has classes at all. A JavaScript compiler typically has to perform a hidden class transform, to extract static layouts from objects. This relies on some dynamic feedback and static inspection (as a first approximation, the fields initialised in a JavaScript constructor can be instance variables and other fields can be added as linked lists). In MysoreScript, classes have a fixed layout defined by their class definition.

Additionally, methods in MysoreScript can not be replaced at run time. This does not make life significantly simpler for an optimiser but does make it slightly easier. If you know the class of an object and the selector, then in MysoreScript you can guarantee that the method lookup that you did last time will be correct the second time (although the compiler may have inserted a new compiled function for the method).

Numbers in MysoreScript are 61-bit integers. Although JavaScript only provides double-precision floating point values at the language level, an efficient implementation needs to use integers for small values and only promote them to doubles on overflow. It also needs to use slightly more complex mechanisms (NaN boxing) for hiding numbers inside object pointers. One possible (simple) extension for MysoreScript would be to add Smalltalk-like integer support, with small integers being silently promoted to arbitrary-precision integers.

MysoreScript also has almost no error checking and lacks a language-level mechanism for sensibly handling errors. There are no exceptions. These involve stack unwinding and could be added in one of two ways to MysoreScript:

• Use the ‘zero-cost’ exception mechanism to allow the generic unwinder to handle them. Possibly also map them to C++ exceptions so that they can be caught easily in the interpreter.

• Make each method and closure return a special value to indicate that an exception has occurred. This is the common case in Java implementations, where exceptions are common.

The lack of error checking also means that poorly formed programs are likely to hit assertions in the interpreter or compiler, rather than helpful error messages. This is not a serious limitation for MysoreScript’s intended use (teaching about the internals of a compiler) but can be problematic if you try to write code using it.

Unlike JavaScript (but like Smalltalk), all fields are private to the object. It is not possible to access the fields of another object without providing accessors. This simplifies parsing, but does not have a significant effect on compilation complexity.

Implementation

Every value is a tagged pointer of a type identified by the low 3 bits:

All objects begin with a pointer to their class. With the exception of String and Closure objects, the size is statically defined by the class. Strings and closures contain a variable number of characters or a variable number of bound objects, respectively.

Closures in MysoreScript are objects. The types of the primitive classes are defined in the runtime.hh file.

MysoreScript includes an AST interpreter and a JIT compiler. The AST interpreter is responsible for invoking the JIT compiler on demand for hot portions of the code. In this implementation, that is done at method granularity.

Methods and Closures

Each method has, as in Objective-C, two hidden first parameters: the receiver (a pointer to the self object) and the selector (cmd). The selector is visible in code as a small integer object, but it is passed as a 32-bit primitive integer. As with Objective-C, the cmd parameter exists so that the called method can tell what the caller thought it was calling. This is currently just used when invoking a method that doesn’t exist (to provide a diagnostic message), but it could be used for forwarding.

Closures just take one hidden parameter, a pointer to the closure object.