janet/doc/The-Janet-Abstract-Machine-Bytecode.md

The Janet language is implemented on top of an abstract machine (AM). The compiler
converts Janet data structures to this bytecode, which can then be efficiently executed
from inside a C program. To understand the janet bytecode, it is useful to understand
the abstractions used inside the Janet AM, as well as the C types used to implement these
features.

## The Stack = The Fiber

A Janet Fiber is the type used to represent multiple concurrent processes
in janet. It is basically a wrapper around the idea of a stack. The stack is
divided into a number of stack frames (`JanetStackFrame *` in C), each of which
contains information such as the function that created the stack frame, 
the program counter for the stack frame, a pointer to the previous frame,
and the size of the frame. Each stack frame also is paired with a number
registers.

```
X: Slot

X
X - Stack Top, for next function call.
-----
Frame next
-----
X
X
X
X
X
X
X - Stack 0
-----
Frame 0
-----
X
X
X - Stack -1
-----
Frame -1
-----
X
X
X
X
X - Stack -2
-----
Frame -2
-----
...
...
...
----- 
Bottom of stack
```

Fibers also have an incomplete stack frame for the next function call on top
of their stacks. Making a function call involves pushing arguments to this
temporary stack, and then invoking either the CALL or TCALL instructions.
Arguments for the next function call are pushed via the PUSH, PUSH2, PUSH3, and
PUSHA instructions. The stack of a fiber will grow as large as needed, although by
default janet will limit the maximum size of a fiber's stack.
The maximum stack size can be modified on a per fiber basis.

The slots in the stack are exposed as virtual registers to instructions. They
can hold any Janet value.

## Closures

All functions in janet are closures; they combine some bytecode instructions
with 0 or more environments. In the C source, a closure (hereby the same as
a function) is represented by the type `JanetFunction *`. The bytecode instruction
part of the function is represented by `JanetFuncDef *`, and a function environment
is represented with `JanetFuncEnv *`.

The function definition part of a function (the 'bytecode' part, `JanetFuncDef *`),
we also store various metadata about the function which is useful for debugging,
as well as constants referenced by the function.

## C Functions

Janet uses C functions to bridge to native code. A C function
(`JanetCFunction *` in C) is a C function pointer that can be called like
a normal janet closure. From the perspective of the bytecode instruction set, there is no difference
in invoking a C function and invoking a normal janet function.

## Bytecode Format

Janet bytecode presents an interface to a virtual machine with a large number
of identical registers that can hold any Janet value (`Janet *` in C). Most instructions
have a destination register, and 1 or 2 source register. Registers are simply
named with positive integers.

Each instruction is a 32 bit integer, meaning that the instruction set is a constant
width RISC instruction set like MIPS. The opcode of each instruction is the least significant
byte of the instruction. The highest bit of
this leading byte is reserved for debugging purpose, so there are 128 possible opcodes encodable
with this scheme. Not all of these possible opcode are defined, and will trap the interpreter
and emit a debug signal. Note that this mean an unknown opcode is still valid bytecode, it will
just put the interpreter into a debug state when executed.

```
X - Payload bits
O - Opcode bits

   4    3    2    1
+----+----+----+----+
| XX | XX | XX | OO |
+----+----+----+----+
```

8 bits for the opcode leaves 24 bits for the payload, which may or may not be utilized.
There are a few instruction variants that divide these payload bits.

* 0 arg - Used for noops, returning nil, or other instructions that take no
  arguments. The payload is essentially ignored.
* 1 arg - All payload bits correspond to a single value, usually a signed or unsigned integer.
  Used for instructions of 1 argument, like returning a value, yielding a value to the parent fiber,
  or doing a (relative) jump.
* 2 arg - Payload is split into byte 2 and bytes 3 and 4.
  The first argument is the 8 bit value from byte 2, and the second argument is the 16 bit value
  from bytes 3 and 4 (`instruction >> 16`). Used for instructions of two arguments, like move, normal
  function calls, conditionals, etc.
* 3 arg - Bytes 2, 3, and 4 each correspond to an 8 bit argument.
  Used for arithmetic operations, emitting a signal, etc.

These instruction variants can be further refined based on the semantics of the arguments.
Some instructions may treat an argument as a slot index, while other instructions
will treat the argument as a signed integer literal, and index for a constant, an index
for an environment, or an unsigned integer.

## Instruction Reference

A listing of all opcode values can be found in src/include/janet/janetopcodes.h. The janet assembly
short names can be found src/assembler/asm.c. In this document, we will refer to the instructions
by their short names as presented to the assembler rather than their numerical values.

Each instruction is also listed with a signature, which are the arguments the instruction
expects. There are a handful of instruction signatures, which combine the arity and type
of the instruction. The assembler does not
do any typechecking per closure, but does prevent jumping to invalid instructions and
failure to return or error.

### Notation

* The $ prefix indicates that a instruction parameter is acting as a virtual register (slot).
  If a parameter does not have the $ suffix in the description, it is acting as some kind
  of literal (usually an unsigned integer for indexes, and a signed integer for literal integers).

* Some operators in the description have the suffix 'i' or 'r'. These indicate
  that these operators correspond to integers or real numbers only, respectively. All
  bitwise operators and bit shifts only work with integers. 

* The `>>>` indicates unsigned right shift, as in Java. Because all integers in janet are
  signed, we differentiate the two kinds of right bit shift.

* The 'im' suffix in the instruction name is short for immediate.

### Reference Table

| Instruction | Signature                   | Description                       |
| ----------- | --------------------------- | --------------------------------- |
| `add`       | `(add dest lhs rhs)`        | $dest = $lhs + $rhs               | 
| `addim`     | `(addim dest lhs im)`       | $dest = $lhs + im                 |
| `band`      | `(band dest lhs rhs)`       | $dest = $lhs & $rhs               |
| `bnot`      | `(bnot dest operand)`       | $dest = ~$operand                 |
| `bor`       | `(bor dest lhs rhs)`        | $dest = $lhs | $rhs               |
| `bxor`      | `(bxor dest lhs rhs)`       | $dest = $lhs ^ $rhs               |
| `call`      | `(call dest callee)`        | $dest = call($callee, args)       |
| `clo`       | `(clo dest index)`          | $dest = closure(defs[$index])     |
| `cmp`       | `(cmp dest lhs rhs)`        | $dest = janet\_compare($lhs, $rhs)|
| `div`       | `(div dest lhs rhs)`        | $dest = $lhs / $rhs               |
| `divim`     | `(divim dest lhs im)`       | $dest = $lhs / im                 |
| `eq`        | `(eq dest lhs rhs)`         | $dest = $lhs == $rhs              |
| `eqim`      | `(eqim dest lhs im)`        | $dest = $lhs == im                |
| `err`       | `(err message)`             | Throw error $message.             |
| `get`       | `(get dest ds key)`         | $dest = $ds[$key]                 |
| `geti`      | `(geti dest ds index)`      | $dest = $ds[index]                |
| `gt`        | `(gt dest lhs rhs)`         | $dest = $lhs \> $rhs              |
| `gtim`      | `(gtim dest lhs im)`        | $dest = $lhs \> im                |
| `jmp`       | `(jmp label)`               | pc = label, pc += offset          |
| `jmpif`     | `(jmpif cond label)`        | if $cond pc = label else pc++     |
| `jmpno`     | `(jmpno cond label)`        | if $cond pc++ else pc = label     |
| `ldc`       | `(ldc dest index)`          | $dest = constants[index]          |
| `ldf`       | `(ldf dest)`                | $dest = false                     |
| `ldi`       | `(ldi dest integer)`        | $dest = integer                   |
| `ldn`       | `(ldn dest)`                | $dest = nil                       |
| `lds`       | `(lds dest)`                | $dest = current closure (self)    |
| `ldt`       | `(ldt dest)`                | $dest = true                      |
| `ldu`       | `(ldu dest env index)`      | $dest = envs[env][index]          |
| `len`       | `(len dest ds)`             | $dest = length(ds)                |
| `lt`        | `(lt dest lhs rhs)`         | $dest = $lhs \< $rhs              |
| `ltim`      | `(ltim dest lhs im)`        | $dest = $lhs \< im                |
| `mkarr`     | `(mkarr dest)`              | $dest = call(array, args)         |
| `mkbuf`     | `(mkbuf dest)`              | $dest = call(buffer, args)        |
| `mktab`     | `(mktab dest)`              | $dest = call(table, args)         |
| `mkstr`     | `(mkstr dest)`              | $dest = call(string, args)        |
| `mkstu`     | `(mkstu dest)`              | $dest = call(struct, args)        |
| `mktup`     | `(mktup dest)`              | $dest = call(tuple, args)         |
| `movf`      | `(movf src dest)`           | $dest = $src                      |
| `movn`      | `(movn dest src)`           | $dest = $src                      |
| `mul`       | `(mul dest lhs rhs)`        | $dest = $lhs \* $rhs              |
| `mulim`     | `(mulim dest lhs im)`       | $dest = $lhs \* im                |
| `noop`      | `(noop)`                    | Does nothing.                     |
| `push`      | `(push val)`                | Push $val on arg                  |
| `push2`     | `(push2 val1 val3)`         | Push $val1, $val2 on args         |
| `push3`     | `(push3 val1 val2 val3)`    | Push $val1, $val2, $val3, on args |
| `pusha`     | `(pusha array)`             | Push values in $array on args     |
| `put`       | `(put ds key val)`          | $ds[$key] = $val                  |
| `puti`      | `(puti ds index val)`       | $ds[index] = $val                 |
| `res`       | `(res dest fiber val)`      | $dest = resume $fiber with $val   |
| `ret`       | `(ret val)`                 | Return $val                       |
| `retn`      | `(retn)`                    | Return nil                        |
| `setu`      | `(setu env index val)`      | envs[env][index] = $val           |
| `sig`       | `(sig dest value sigtype)`  | $dest = emit $value as sigtype    |
| `sl`        | `(sl dest lhs rhs)`         | $dest = $lhs << $rhs              |
| `slim`      | `(slim dest lhs shamt)`     | $dest = $lhs << shamt             |
| `sr`        | `(sr dest lhs rhs)`         | $dest = $lhs >> $rhs              |
| `srim`      | `(srim dest lhs shamt)`     | $dest = $lhs >> shamt             |
| `sru`       | `(sru dest lhs rhs)`        | $dest = $lhs >>> $rhs             |
| `sruim`     | `(sruim dest lhs shamt)`    | $dest = $lhs >>> shamt            |
| `sub`       | `(sub dest lhs rhs)`        | $dest = $lhs - $rhs               |
| `tcall`     | `(tcall callee)`            | Return call($callee, args)        |
| `tchck`     | `(tcheck slot types)`       | Assert $slot does matches types   |