brili.ts

#!/usr/bin/env node
import internal = require('stream');
import * as bril from './bril';
import { readStdin, unreachable } from './util';

/**
 * An interpreter error to print to the console.
 */
class BriliError extends Error {
  constructor(message?: string) {
    super(message);
    Object.setPrototypeOf(this, new.target.prototype);
    this.name = BriliError.name;
  }
}

/**
 * Create an interpreter error object to throw.
 */
function error(message: string): BriliError {
  return new BriliError(message);
}

/**
 * An abstract key class used to access the heap.
 * This allows for "pointer arithmetic" on keys,
 * while still allowing lookups based on the based pointer of each allocation.
 */
export class Key {
  readonly base: number;
  readonly offset: number;

  constructor(b: number, o: number) {
    this.base = b;
    this.offset = o;
  }

  add(offset: number) {
    return new Key(this.base, this.offset + offset);
  }
}

/**
 * A Heap maps Keys to arrays of a given type.
 */
export class Heap<X> {

  private readonly storage: Map<number, X[]>
  constructor() {
    this.storage = new Map()
  }

  isEmpty(): boolean {
    return this.storage.size == 0;
  }

  private count = 0;
  private getNewBase(): number {
    let val = this.count;
    this.count++;
    return val;
  }

  private freeKey(key: Key) {
    return;
  }

  alloc(amt: number): Key {
    if (amt <= 0) {
      throw error(`cannot allocate ${amt} entries`);
    }
    let base = this.getNewBase();
    this.storage.set(base, new Array(amt))
    return new Key(base, 0);
  }

  free(key: Key) {
    if (this.storage.has(key.base) && key.offset == 0) {
      this.freeKey(key);
      this.storage.delete(key.base);
    } else {
      throw error(`Tried to free illegal memory location base: ${key.base}, offset: ${key.offset}. Offset must be 0.`);
    }
  }

  write(key: Key, val: X) {
    let data = this.storage.get(key.base);
    if (data && data.length > key.offset && key.offset >= 0) {
      data[key.offset] = val;
    } else {
      throw error(`Uninitialized heap location ${key.base} and/or illegal offset ${key.offset}`);
    }
  }

  read(key: Key): X {
    let data = this.storage.get(key.base);
    if (data && data.length > key.offset && key.offset >= 0) {
      return data[key.offset];
    } else {
      throw error(`Uninitialized heap location ${key.base} and/or illegal offset ${key.offset}`);
    }
  }
}

class GarbageCollector {

  private referenceCounts: Map<Key, number>
  private heap: Heap<Value>

  constructor(heap: Heap<Value>) {
    this.referenceCounts = new Map()
    this.heap = heap
  }

  alloc(key: Key) {
    this.referenceCounts.set(key, 0)
  }

  assign(env: Env, dest: bril.Ident, key: Key) {
    let value = env.get(dest)
    if (value != undefined) {
      this.dec((value as Pointer).loc)
    }
    this.inc(key)
  }

  inc(key: Key) {
    let count = this.referenceCounts.get(key)
    // count is a number or undefined (if key does not exist)
    if (count != undefined) {
      this.referenceCounts.set(key, count + 1)
    }
  }

  dec(key: Key) {
    let count = this.referenceCounts.get(key)
    // count is a number or undefined (if key does not exist)
    if (count != undefined) {
      this.referenceCounts.set(key, count - 1)
      if (count == 1) {
        // new count is 0, so free this key
        this.heap.free(key);
        this.referenceCounts.delete(key)
      }
    }
  }

  incCounts(env: Env) {
    env.forEach((value, _) => {
      if (value.hasOwnProperty("loc")) {
        this.inc((value as Pointer).loc);
      }
    })
  }

  decCounts(env: Env) {
    env.forEach((value, _) => {
      if (value.hasOwnProperty("loc")) {
        this.dec((value as Pointer).loc)
      }
    })
  }

}

const argCounts: { [key in bril.OpCode]: number | null } = {
  add: 2,
  mul: 2,
  sub: 2,
  div: 2,
  id: 1,
  lt: 2,
  le: 2,
  gt: 2,
  ge: 2,
  eq: 2,
  not: 1,
  and: 2,
  or: 2,
  fadd: 2,
  fmul: 2,
  fsub: 2,
  fdiv: 2,
  flt: 2,
  fle: 2,
  fgt: 2,
  fge: 2,
  feq: 2,
  print: null,  // Any number of arguments.
  br: 1,
  jmp: 0,
  ret: null,  // (Should be 0 or 1.)
  nop: 0,
  call: null,
  alloc: 1,
  free: 1,
  store: 2,
  load: 1,
  ptradd: 2,
  phi: null,
  speculate: 0,
  guard: 1,
  commit: 0,
};

type Pointer = {
  loc: Key;
  type: bril.Type;
}

type Value = boolean | BigInt | Pointer | number;
type Env = Map<bril.Ident, Value>;

/**
 * Check whether a run-time value matches the given static type.
 */
function typeCheck(val: Value, typ: bril.Type): boolean {
  if (typ === "int") {
    return typeof val === "bigint";
  } else if (typ === "bool") {
    return typeof val === "boolean";
  } else if (typ === "float") {
    return typeof val === "number";
  } else if (typeof typ === "object" && typ.hasOwnProperty("ptr")) {
    return val.hasOwnProperty("loc");
  }
  throw error(`unknown type ${typ}`);
}

/**
 * Check whether the types are equal.
 */
function typeCmp(lhs: bril.Type, rhs: bril.Type): boolean {
  if (lhs === "int" || lhs == "bool" || lhs == "float") {
    return lhs == rhs;
  } else {
    if (typeof rhs === "object" && rhs.hasOwnProperty("ptr")) {
      return typeCmp(lhs.ptr, rhs.ptr);
    } else {
      return false;
    }
  }
}

function get(env: Env, ident: bril.Ident) {
  let val = env.get(ident);
  if (typeof val === 'undefined') {
    throw error(`undefined variable ${ident}`);
  }
  return val;
}

function findFunc(func: bril.Ident, funcs: readonly bril.Function[]) {
  let matches = funcs.filter(function (f: bril.Function) {
    return f.name === func;
  });

  if (matches.length == 0) {
    throw error(`no function of name ${func} found`);
  } else if (matches.length > 1) {
    throw error(`multiple functions of name ${func} found`);
  }

  return matches[0];
}

function alloc(ptrType: bril.ParamType, amt: number, heap: Heap<Value>): Pointer {
  if (typeof ptrType != 'object') {
    throw error(`unspecified pointer type ${ptrType}`);
  } else if (amt <= 0) {
    throw error(`must allocate a positive amount of memory: ${amt} <= 0`);
  } else {
    let loc = heap.alloc(amt)
    let dataType = ptrType.ptr;
    return {
      loc: loc,
      type: dataType
    }
  }
}

/**
 * Ensure that the instruction has exactly `count` arguments,
 * throw an exception otherwise.
 */
function checkArgs(instr: bril.Operation, count: number) {
  let found = instr.args ? instr.args.length : 0;
  if (found != count) {
    throw error(`${instr.op} takes ${count} argument(s); got ${found}`);
  }
}

function getPtr(instr: bril.Operation, env: Env, index: number): Pointer {
  let val = getArgument(instr, env, index);
  if (typeof val !== 'object' || val instanceof BigInt) {
    throw `${instr.op} argument ${index} must be a Pointer`;
  }
  return val;
}

function getArgument(instr: bril.Operation, env: Env, index: number, typ?: bril.Type) {
  let args = instr.args || [];
  if (args.length <= index) {
    throw error(`${instr.op} expected at least ${index + 1} arguments; got ${args.length}`);
  }
  let val = get(env, args[index]);
  if (typ && !typeCheck(val, typ)) {
    throw error(`${instr.op} argument ${index} must be a ${typ}`);
  }
  return val;
}

function getInt(instr: bril.Operation, env: Env, index: number): bigint {
  return getArgument(instr, env, index, 'int') as bigint;
}

function getBool(instr: bril.Operation, env: Env, index: number): boolean {
  return getArgument(instr, env, index, 'bool') as boolean;
}

function getFloat(instr: bril.Operation, env: Env, index: number): number {
  return getArgument(instr, env, index, 'float') as number;
}

function getLabel(instr: bril.Operation, index: number): bril.Ident {
  if (!instr.labels) {
    throw error(`missing labels; expected at least ${index + 1}`);
  }
  if (instr.labels.length <= index) {
    throw error(`expecting ${index + 1} labels; found ${instr.labels.length}`);
  }
  return instr.labels[index];
}

function getFunc(instr: bril.Operation, index: number): bril.Ident {
  if (!instr.funcs) {
    throw error(`missing functions; expected at least ${index + 1}`);
  }
  if (instr.funcs.length <= index) {
    throw error(`expecting ${index + 1} functions; found ${instr.funcs.length}`);
  }
  return instr.funcs[index];
}

/**
 * The thing to do after interpreting an instruction: this is how `evalInstr`
 * communicates control-flow actions back to the top-level interpreter loop.
 */
type Action =
  { "action": "next" } |  // Normal execution: just proceed to next instruction.
  { "action": "jump", "label": bril.Ident } |
  { "action": "end", "ret": Value | null } |
  { "action": "speculate" } |
  { "action": "commit" } |
  { "action": "abort", "label": bril.Ident };
let NEXT: Action = { "action": "next" };

/**
 * The interpreter state that's threaded through recursive calls.
 */
type State = {
  env: Env,
  readonly heap: Heap<Value>,
  readonly funcs: readonly bril.Function[],

  // For profiling: a total count of the number of instructions executed.
  icount: bigint,

  // For SSA (phi-node) execution: keep track of recently-seen labels.j
  curlabel: string | null,
  lastlabel: string | null,

  // For speculation: the state at the point where speculation began.
  specparent: State | null,

  // For garbage collection: the state of the RC garbage collector
  gc: GarbageCollector,

  // For tracing: indicates whether or not to trace
  shouldTrace: boolean,

  // For tracing: stores the header of the current trace
  traceHeader: string | null,

  // For tracing: labels encountered
  seen: Array<String>,
}

/**
 * Interpet a call instruction.
 */
function evalCall(instr: bril.Operation, state: State): Action {
  // Which function are we calling?
  let funcName = getFunc(instr, 0);
  let func = findFunc(funcName, state.funcs);
  if (func === null) {
    throw error(`undefined function ${funcName}`);
  }

  let newEnv: Env = new Map();

  // Check arity of arguments and definition.
  let params = func.args || [];
  let args = instr.args || [];
  if (params.length !== args.length) {
    throw error(`function expected ${params.length} arguments, got ${args.length}`);
  }

  for (let i = 0; i < params.length; i++) {
    // Look up the variable in the current (calling) environment.
    let value = get(state.env, args[i]);

    // Check argument types
    if (!typeCheck(value, params[i].type)) {
      throw error(`function argument type mismatch`);
    }

    // Set the value of the arg in the new (function) environment.
    newEnv.set(params[i].name, value);
  }

  // Invoke the interpreter on the function.
  let newState: State = {
    env: newEnv,
    heap: state.heap,
    funcs: state.funcs,
    icount: state.icount,
    lastlabel: null,
    curlabel: null,
    specparent: null,  // Speculation not allowed.
    gc: state.gc,
    shouldTrace: state.shouldTrace,
    traceHeader: state.traceHeader,
    seen: state.seen,
  }
  // Increment all reference counts in newEnv before function call 
  newState.gc.incCounts(newEnv)
  let retVal = evalFunc(func, newState);
  state.icount = newState.icount;

  // Dynamically check the function's return value and type.
  if (!('dest' in instr)) {  // `instr` is an `EffectOperation`.
    // Expected void function
    if (retVal !== null) {
      throw error(`unexpected value returned without destination`);
    }
    if (func.type !== undefined) {
      throw error(`non-void function (type: ${func.type}) doesn't return anything`);
    }
  } else {  // `instr` is a `ValueOperation`.
    // Expected non-void function
    if (instr.type === undefined) {
      throw error(`function call must include a type if it has a destination`);
    }
    if (instr.dest === undefined) {
      throw error(`function call must include a destination if it has a type`);
    }
    if (retVal === null) {
      throw error(`non-void function (type: ${func.type}) doesn't return anything`);
    }
    if (!typeCheck(retVal, instr.type)) {
      throw error(`type of value returned by function does not match destination type`);
    }
    if (func.type === undefined) {
      throw error(`function with void return type used in value call`);
    }
    if (!typeCmp(instr.type, func.type)) {
      throw error(`type of value returned by function does not match declaration`);
    }
    if (retVal.hasOwnProperty("loc")) {
      // if the return value is a pointer, assign the instruction dest to this loc
      state.gc.assign(state.env, instr.dest, (retVal as Pointer).loc)
    }
    state.env.set(instr.dest, retVal);
  }
  // Increment reference counts after function call 
  state.gc.decCounts(newEnv)
  return NEXT;
}

function handleLabel(label: string, state: State) {
  if (state.traceHeader == null) {
    // begin tracing here
    state.traceHeader = label
    state.seen.push(label)
    state.shouldTrace = true
  }
  else if (state.seen.indexOf(label) > -1) {
    // we've hit a loop, so we stop tracing
    if (state.shouldTrace) {
      endTrace(state)
    }
  }
  else {
    // add this label to the labels seen
    state.seen.push(label)
  }
}

function printTrace(instr: bril.Instruction) {
  console.log(JSON.stringify(instr))
}

function endTrace(state: State) {
  state.shouldTrace = false
  let commit: bril.Instruction = { op: "commit" }
  printTrace(commit)
}

/**
 * It prints out the instruction, and then it ends the trace if the instruction is
 * a call, return, jump, or memory instruction. Otherwise, it replaces branches
 * with guard instructions
 * @param instr - bril.Instruction
 * @param {State} state - The current state of the trace.
 * @returns The function traceInstr is being returned.
 */
function traceInstr(instr: bril.Instruction, state: State) {

  switch (instr.op) {
    case "const":
    case "id":
    case "add":
    case "mul":
    case "sub":
    case "div":
    case "le":
    case "lt":
    case "gt":
    case "ge":
    case "eq":
    case "not":
    case "and":
    case "or":
    case "fadd":
    case "fsub":
    case "fmul":
    case "fdiv":
    case "fle":
    case "flt":
    case "fgt":
    case "fge":
    case "feq":
    case "print":
    case "phi":
    case "ptradd": {
      printTrace(instr)
      return
    }

    case "nop": {
      return
    }

    case "speculate":
    case "guard":
    case "commit": {
      // bail out on speculation instructions
      endTrace(state)
      return
    }

    case "alloc":
    case "free":
    case "store":
    case "load": {
      // bail out on memory instructions
      endTrace(state)
      return
    }

    case "ret":
    case "call": {
      // bail out on calls and returns
      endTrace(state)
      return
    }

    case "jmp": {
      // eliminate jumps
      return
    }

    case "br": {
      // replace branches with guard instructions
      let header = state.traceHeader
      if (header) {
        let guard: bril.Instruction = { op: "guard", args: instr.args, labels: [header] }
        printTrace(guard)
      }
      return
    }
  }
}

/**
 * Interpret an instruction in a given environment, possibly updating the
 * environment. If the instruction branches to a new label, return that label;
 * otherwise, return "next" to indicate that we should proceed to the next
 * instruction or "end" to terminate the function.
 */
function evalInstr(instr: bril.Instruction, state: State): Action {

  // instr is some json format instr- figure out how to get it out of the 
  // interpreter and into a trace you can optimize
  // can write to a file, print to the console, etc
  // Figure out when to start and stop tracing...
  // const file = readFileSync('./trace.json', 'utf-8');
  // var file = new File(["trace"], "trace.txt", {
  //   type: "text/plain",
  // });

  state.icount += BigInt(1);

  // Check that we have the right number of arguments.
  if (instr.op !== "const") {
    let count = argCounts[instr.op];
    if (count === undefined) {
      throw error("unknown opcode " + instr.op);
    } else if (count !== null) {
      checkArgs(instr, count);
    }
  }

  // Function calls are not (currently) supported during speculation.
  // It would be cool to add, but aborting from inside a function call
  // would require explicit stack management.
  if (state.specparent && ['call', 'ret'].includes(instr.op)) {
    throw error(`${instr.op} not allowed during speculation`);
  }

  if (state.shouldTrace) {
    traceInstr(instr, state)
  }

  switch (instr.op) {
    case "const":
      // Interpret JSON numbers as either ints or floats.
      let value: Value;
      if (typeof instr.value === "number") {
        if (instr.type === "float")
          value = instr.value;
        else
          value = BigInt(Math.floor(instr.value))
      } else {
        value = instr.value;
      }

      state.env.set(instr.dest, value);
      return NEXT;

    case "id": {
      let val = getArgument(instr, state.env, 0);
      if (val.hasOwnProperty("loc")) {
        state.gc.assign(state.env, instr.dest, (val as Pointer).loc);
      }
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "add": {
      let val = getInt(instr, state.env, 0) + getInt(instr, state.env, 1);
      val = BigInt.asIntN(64, val);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "mul": {
      let val = getInt(instr, state.env, 0) * getInt(instr, state.env, 1);
      val = BigInt.asIntN(64, val);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "sub": {
      let val = getInt(instr, state.env, 0) - getInt(instr, state.env, 1);
      val = BigInt.asIntN(64, val);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "div": {
      let val = getInt(instr, state.env, 0) / getInt(instr, state.env, 1);
      val = BigInt.asIntN(64, val);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "le": {
      let val = getInt(instr, state.env, 0) <= getInt(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "lt": {
      let val = getInt(instr, state.env, 0) < getInt(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "gt": {
      let val = getInt(instr, state.env, 0) > getInt(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "ge": {
      let val = getInt(instr, state.env, 0) >= getInt(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "eq": {
      let val = getInt(instr, state.env, 0) === getInt(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "not": {
      let val = !getBool(instr, state.env, 0);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "and": {
      let val = getBool(instr, state.env, 0) && getBool(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "or": {
      let val = getBool(instr, state.env, 0) || getBool(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "fadd": {
      let val = getFloat(instr, state.env, 0) + getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "fsub": {
      let val = getFloat(instr, state.env, 0) - getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "fmul": {
      let val = getFloat(instr, state.env, 0) * getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "fdiv": {
      let val = getFloat(instr, state.env, 0) / getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "fle": {
      let val = getFloat(instr, state.env, 0) <= getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "flt": {
      let val = getFloat(instr, state.env, 0) < getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "fgt": {
      let val = getFloat(instr, state.env, 0) > getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "fge": {
      let val = getFloat(instr, state.env, 0) >= getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "feq": {
      let val = getFloat(instr, state.env, 0) === getFloat(instr, state.env, 1);
      state.env.set(instr.dest, val);
      return NEXT;
    }

    case "print": {
      let args = instr.args || [];
      let values = args.map(i => get(state.env, i).toString());
      console.log(...values);
      return NEXT;
    }

    case "jmp": {
      return { "action": "jump", "label": getLabel(instr, 0) };
    }

    case "br": {
      let cond = getBool(instr, state.env, 0);
      if (cond) {
        return { "action": "jump", "label": getLabel(instr, 0) };
      } else {
        return { "action": "jump", "label": getLabel(instr, 1) };
      }
    }

    case "ret": {
      let args = instr.args || [];
      if (args.length == 0) {
        return { "action": "end", "ret": null };
      } else if (args.length == 1) {
        let val = get(state.env, args[0]);
        return { "action": "end", "ret": val };
      } else {
        throw error(`ret takes 0 or 1 argument(s); got ${args.length}`);
      }
    }

    case "nop": {
      return NEXT;
    }

    case "call": {
      return evalCall(instr, state);
    }

    case "alloc": {
      let amt = getInt(instr, state.env, 0);
      let typ = instr.type;
      if (!(typeof typ === "object" && typ.hasOwnProperty('ptr'))) {
        throw error(`cannot allocate non-pointer type ${instr.type}`);
      }
      let ptr = alloc(typ, Number(amt), state.heap);
      let ptrLoc = ptr.loc
      state.gc.alloc(ptrLoc);
      state.gc.assign(state.env, instr.dest, ptrLoc)
      state.env.set(instr.dest, ptr);
      return NEXT;
    }

    case "free": {
      // nop
      return NEXT;
    }

    case "store": {
      let target = getPtr(instr, state.env, 0);
      switch (target.type) {
        case "int": {
          state.heap.write(target.loc, getInt(instr, state.env, 1));
          break;
        }
        case "bool": {
          state.heap.write(target.loc, getBool(instr, state.env, 1));
          break;
        }
        default: {
          state.heap.write(target.loc, getPtr(instr, state.env, 1));
          break;
        }
      }
      return NEXT;
    }

    case "load": {
      let ptr = getPtr(instr, state.env, 0);
      let val = state.heap.read(ptr.loc);
      if (val === undefined || val === null) {
        throw error(`Pointer ${instr.args![0]} points to uninitialized data`);
      } else {
        state.env.set(instr.dest, val);
      }
      return NEXT;
    }

    case "ptradd": {
      let ptr = getPtr(instr, state.env, 0)
      let val = getInt(instr, state.env, 1)
      let newPtr = ptr.loc.add(Number(val))
      state.gc.assign(state.env, instr.dest, newPtr)
      state.env.set(instr.dest, { loc: newPtr, type: ptr.type })
      return NEXT;
    }

    case "phi": {
      let labels = instr.labels || [];
      let args = instr.args || [];
      if (labels.length != args.length) {
        throw error(`phi node has unequal numbers of labels and args`);
      }
      if (!state.lastlabel) {
        throw error(`phi node executed with no last label`);
      }
      let idx = labels.indexOf(state.lastlabel);
      if (idx === -1) {
        // Last label not handled. Leave uninitialized.
        state.env.delete(instr.dest);
      } else {
        // Copy the right argument (including an undefined one).
        if (!instr.args || idx >= instr.args.length) {
          throw error(`phi node needed at least ${idx + 1} arguments`);
        }
        let src = instr.args[idx];
        let val = state.env.get(src);
        if (val === undefined) {
          state.env.delete(instr.dest);
        } else {
          state.env.set(instr.dest, val);
        }
      }
      return NEXT;
    }

    // Begin speculation.
    case "speculate": {
      return { "action": "speculate" };
    }

    // Abort speculation if the condition is false.
    case "guard": {
      if (getBool(instr, state.env, 0)) {
        return NEXT;
      } else {
        return { "action": "abort", "label": getLabel(instr, 0) };
      }
    }

    // Resolve speculation, making speculative state real.
    case "commit": {
      return { "action": "commit" };
    }

  }
  unreachable(instr);
  throw error(`unhandled opcode ${(instr as any).op}`);
}

function evalFunc(func: bril.Function, state: State): Value | null {
  for (let i = 0; i < func.instrs.length; ++i) {
    let line = func.instrs[i];
    if ('op' in line) {
      // Run an instruction.
      let action = evalInstr(line, state);

      // Take the prescribed action.
      switch (action.action) {
        case 'end': {
          // Return from this function.
          return action.ret;
        }
        case 'speculate': {
          // Begin speculation.
          state.specparent = { ...state };
          state.env = new Map(state.env);
          break;
        }
        case 'commit': {
          // Resolve speculation.
          if (!state.specparent) {
            throw error(`commit in non-speculative state`);
          }
          state.specparent = null;
          break;
        }
        case 'abort': {
          // Restore state.
          if (!state.specparent) {
            throw error(`abort in non-speculative state`);
          }
          // We do *not* restore `icount` from the saved state to ensure that we
          // count "aborted" instructions.
          Object.assign(state, {
            env: state.specparent.env,
            lastlabel: state.specparent.lastlabel,
            curlabel: state.specparent.curlabel,
            specparent: state.specparent.specparent,
          });
          break;
        }
        case 'next':
        case 'jump':
          break;
        default:
          unreachable(action);
          throw error(`unhandled action ${(action as any).action}`);
      }
      // Move to a label.
      if ('label' in action) {
        // Search for the label and transfer control.
        for (i = 0; i < func.instrs.length; ++i) {
          let sLine = func.instrs[i];
          if ('label' in sLine && sLine.label === action.label) {
            --i;  // Execute the label next.
            break;
          }
        }
        if (i === func.instrs.length) {
          throw error(`label ${action.label} not found`);
        }
      }
    } else if ('label' in line) {
      // Update CFG tracking for SSA phi nodes.
      state.lastlabel = state.curlabel;
      state.curlabel = line.label;

      if (state.shouldTrace) {
        handleLabel(line.label, state)
      }
    }
  }

  // Reached the end of the function without hitting `ret`.
  if (state.specparent) {
    throw error(`implicit return in speculative state`);
  }
  return null;
}

function parseBool(s: string): boolean {
  if (s === 'true') {
    return true;
  } else if (s === 'false') {
    return false;
  } else {
    throw error(`boolean argument to main must be 'true'/'false'; got ${s}`);
  }
}

function parseMainArguments(expected: bril.Argument[], args: string[]): Env {
  let newEnv: Env = new Map();

  if (args.length !== expected.length) {
    throw error(`mismatched main argument arity: expected ${expected.length}; got ${args.length}`);
  }

  for (let i = 0; i < args.length; i++) {
    let type = expected[i].type;
    switch (type) {
      case "int":
        let n: bigint = BigInt(parseInt(args[i]));
        newEnv.set(expected[i].name, n as Value);
        break;
      case "bool":
        let b: boolean = parseBool(args[i]);
        newEnv.set(expected[i].name, b as Value);
        break;
    }
  }
  return newEnv;
}

function evalProg(prog: bril.Program) {
  let heap = new Heap<Value>()
  let main = findFunc("main", prog.functions);
  if (main === null) {
    console.warn(`no main function defined, doing nothing`);
    return;
  }

  // Silly argument parsing to find the `-p` flag.
  let args: string[] = process.argv.slice(2, process.argv.length);
  let profiling = false;
  let pidx = args.indexOf('-p');
  if (pidx > -1) {
    profiling = true;
    args.splice(pidx, 1);
  }
  let tracing = false;
  let tidx = args.indexOf("-t");
  if (tidx > -1) {
    tracing = true;
    args.splice(tidx, 1);
  }

  // Remaining arguments are for the main function.k
  let expected = main.args || [];
  let newEnv = parseMainArguments(expected, args);

  let state: State = {
    funcs: prog.functions,
    heap,
    env: newEnv,
    icount: BigInt(0),
    lastlabel: null,
    curlabel: null,
    specparent: null,
    gc: new GarbageCollector(heap),
    shouldTrace: tracing,
    traceHeader: null,
    seen: [],
  }
  state.gc.incCounts(state.env)
  evalFunc(main, state);
  state.gc.decCounts(state.env)

  if (!heap.isEmpty()) {
    throw error(`Some memory locations have not been freed by end of execution.`);
  }

  if (profiling) {
    console.error(`total_dyn_inst: ${state.icount}`);
  }

}

async function main() {
  try {
    let prog = JSON.parse(await readStdin()) as bril.Program;
    evalProg(prog);
  }
  catch (e) {
    if (e instanceof BriliError) {
      console.error(`error: ${e.message}`);
      process.exit(2);
    } else {
      throw e;
    }
  }
}

// Make unhandled promise rejections terminate.
process.on('unhandledRejection', e => { throw e });

main();