rubyx/lib/vm/method_compiler/README.md

### Compiling

The typed syntax tree is created by the ruby compiler.

The code in this directory compiles the typed tree to the register machine code, and
Parfait object structure.

If this were an interpreter, we would just walk the tree and do what it says.
Since it's not things are a little more difficult, especially in time.

When compiling we deal with two times, compile-time and run-time.
All the headache comes from mixing those two up.*

Similarly, the result of compiling is two-fold: a static and a dynamic part.

- the static part are objects like the constants, but also defined classes and their methods
- the dynamic part is the code, which is stored as streams of instructions in the MethodSource

Too make things a little simpler, we create a very high level instruction stream at first and then
run transformation and optimization passes on the stream to improve it.

The compiler has a method for each class of typed tree, named along on_xxx with xxx as the type

#### Compiler holds scope

The Compiler instance can hold arbitrary scope needed during the compilation.
A class statement sets the current @type scope , a method definition the @method.
If either are not set when needed compile errors will follow. So easy, so nice.

All code is encoded as a stream of Instructions in the MethodSource.
Instructions are stored as a list of Blocks, and Blocks are the smallest unit of code,
which is always linear.

Code is added to the method (using add_code), rather than working with the actual instructions.
This is so each compiling method can just do it's bit and be unaware of the larger structure
that is being created.
The general structure of the instructions is a graph
(with if's and whiles and breaks and what), but we build it to have one start and *one* end (return).


#### Messages and frames

Since the machine is oo we define it in objects.

Also it is important to define how instructions operate, which is is in a physical machine would
be by changing the contents of registers or some stack.

Our machine is not a register machine, but an object machine: it operates directly on objects and
also has no separate stack, only objects. There is only one object which is accessible,
basically meaning pinned to a register, the Message.

One can think of the Message as an oo replacement of the stack.

When a TypedMethod needs to make a call, it creates a NewMessage object.
Messages contain return addresses (yes, plural) and arguments.

The important thing here is that Messages and Frames are normal objects.