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//! Opcodes and the encoding of instructions into bytes.
use super::Opr24;
/// A VM opcode.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum Opcode {
/// Doesn't do anything. Used as a default zero value if something goes wrong.
/// Also used for backpatching purposes.
#[allow(unused)]
Nop,
/// Pushes `nil` onto the stack.
PushNil,
/// Pushes `true` onto the stack.
PushTrue,
/// Pushes `false` onto the stack.
PushFalse,
/// Pushes a number onto the stack. Must be followed by an f64.
PushNumber,
/// Pushes a string onto the stack. Must be followed by a string.
PushString,
/// Creates a closure from the function with the given ID and pushes it onto the stack.
CreateClosure,
/// Creates a unique type that can be later implemented. Must be followed by a string
/// indicating the type's name.
CreateType,
/// Creates a struct instance from the type at the top of the stack, with the specified amount
/// of fields.
CreateStruct,
/// Creates an instance of the trait with the given ID and pushes it onto the stack.
CreateTrait,
/// Creates a list from `operand` values that are at the top of the stack.
CreateList,
/// Creates a dict from `operand * 2` values that are at the top of the stack. The values have
/// to be arranged in `key, value, key, value...` order, from bottom to top.
CreateDict,
/// Assigns the value at the top of the stack to a global. The value stays on the stack.
AssignGlobal,
/// Sinks the value at the top of the stack to a global. The value is consumed.
SinkGlobal,
/// Loads a value from a global.
GetGlobal,
/// Assigns the value at the top of the stack to a local. The value stays on the stack.
AssignLocal,
/// Sinks the value at the top of the stack to a local. The value is consumed.
SinkLocal,
/// Loads a value from a local.
GetLocal,
/// Assigns the value at the top of the stack to an upvalue. The value stays on the stack.
AssignUpvalue,
/// Sinks the value at the top of the stack to an upvalue. The value is consumed.
SinkUpvalue,
/// Loads a value from an upvalue.
GetUpvalue,
/// Closes a local in its upvalue.
CloseLocal,
/// Assigns to a field in the struct on the top of the stack. The struct is consumed but the
/// value remains on the stack.
/// Assumes the second value from top is a struct and not something else.
AssignField,
/// Sinks to a field in the struct on the top of the stack. Both the struct and the value are
/// consumed.
SinkField,
/// Loads a field from the struct on the top of the stack.
/// Assumes the value on top is a struct and not something else.
GetField,
/// Swaps the two values at the top of the stack.
Swap,
/// Removes the value at the top of the stack.
Discard,
// Note that due to how the VM increments the program counter, forward jump instructions as if
// 4 bytes were jumped over implicitly (the actual number of bytes that is jumped over is
// `operand + 4`).
/// Jumps the program counter forward by an amount of bytes.
JumpForward,
/// Jumps the program counter forward by an amount of bytes if the value at the top of the
/// stack is falsy.
JumpForwardIfFalsy,
/// Jumps the program counter forward by an amount of bytes if the value at the top of the
/// stack is truthy.
JumpForwardIfTruthy,
/// Jumps the program counter backward by an amount of bytes.
/// Due to how the VM increments the program counter, the actual amount is `operand - 4`.
JumpBackward,
/// Enters a breakable block by pushing the break sentinel value onto the stack.
EnterBreakableBlock,
/// Exits the n-th breakable block (counted from innermost) by popping values off the stack
/// until `.0` sentinels are removed.
ExitBreakableBlock,
/// Calls a function with `.0` arguments.
Call,
/// Calls the `n`th method with `a` arguments, where `a` is encoded in the lower 8 bits, and
/// `n` is encoded in the upper 16 bits of `.0`.
CallMethod,
/// Returns to the calling function.
Return,
/// Implements a struct according to a prototype identified by the operand.
Implement,
/// Negates a number (prefix `-`).
Negate,
/// Adds two numbers together (infix `+`).
Add,
/// Subtracts a number from another number (infix `-`).
Subtract,
/// Multiplies two numbers together (infix `*`).
Multiply,
/// Divides a number by another number (infix `/`).
Divide,
/// Flips a boolean-like value (truthy values become `false` and falsy values become `true`).
Not,
/// Compares two values for equality.
Equal,
/// Compares two values for less-than relation.
Less,
/// Compares two values for less-than-or-equal relation.
LessEqual,
/// Halts the interpreter loop.
Halt,
}
/// A jump was constructed whose offset stretched too far.
#[derive(Debug)]
pub struct JumpTooFar(());
impl Opcode {
/// The size of an instruction (1 byte opcode + 3 bytes operand).
pub const INSTRUCTION_SIZE: usize = 4;
/// Returns the offset of a forward jump instruction.
fn forward_jump_offset(from: usize, to: usize) -> Result<Opr24, JumpTooFar> {
assert!(to >= from);
let offset = to - from - Self::INSTRUCTION_SIZE;
if u32::try_from(offset).is_err() {
return Err(JumpTooFar(()));
}
Opr24::new(offset as u32).map_err(|_| JumpTooFar(()))
}
/// Constructs a `JumpForward` instruction.
pub fn jump_forward(from: usize, to: usize) -> Result<(Self, Opr24), JumpTooFar> {
let offset = Self::forward_jump_offset(from, to)?;
Ok((Self::JumpForward, offset))
}
/// Constructs a `JumpForwardIfFalsy` instruction.
pub fn jump_forward_if_falsy(from: usize, to: usize) -> Result<(Self, Opr24), JumpTooFar> {
let offset = Self::forward_jump_offset(from, to)?;
Ok((Self::JumpForwardIfFalsy, offset))
}
/// Constructs a `JumpForwardIfTruthy` instruction.
pub fn jump_forward_if_truthy(from: usize, to: usize) -> Result<(Self, Opr24), JumpTooFar> {
let offset = Self::forward_jump_offset(from, to)?;
Ok((Self::JumpForwardIfTruthy, offset))
}
/// Returns the offset of a backward jump instruction.
fn backward_jump_offset(from: usize, to: usize) -> Result<Opr24, JumpTooFar> {
assert!(to <= from);
let offset = from - to + Self::INSTRUCTION_SIZE;
if u32::try_from(offset).is_err() {
return Err(JumpTooFar(()));
}
Opr24::new(offset as u32).map_err(|_| JumpTooFar(()))
}
/// Constructs a `JumpBackward` instruction.
pub fn jump_backward(from: usize, to: usize) -> Result<(Self, Opr24), JumpTooFar> {
let offset = Self::backward_jump_offset(from, to)?;
Ok((Self::JumpBackward, offset))
}
}
/// Types that can be encoded into bytecode.
pub trait EncodeInstruction {
fn encode_instruction(&self) -> [u8; Opcode::INSTRUCTION_SIZE];
}
impl EncodeInstruction for (Opcode, Opr24) {
fn encode_instruction(&self) -> [u8; Opcode::INSTRUCTION_SIZE] {
[self.0 as u8, self.1.bytes[0], self.1.bytes[1], self.1.bytes[2]]
}
}
impl EncodeInstruction for (Opcode, u16) {
fn encode_instruction(&self) -> [u8; Opcode::INSTRUCTION_SIZE] {
let ubytes = self.1.to_le_bytes();
[self.0 as u8, ubytes[0], ubytes[1], 0]
}
}
impl EncodeInstruction for Opcode {
fn encode_instruction(&self) -> [u8; Opcode::INSTRUCTION_SIZE] {
(*self, Opr24 { bytes: [0, 0, 0] }).encode_instruction()
}
}