chibicc/parse.c

// This file contains a recursive descent parser for C.
//
// Most functions in this file are named after the symbols they are
// supposed to read from an input token list. For example, stmt() is
// responsible for reading a statement from a token list. The function
// then construct an AST node representing a statement.
//
// Each function conceptually returns two values, an AST node and
// remaining part of the input tokens. Since C doesn't support
// multiple return values, the remaining tokens are returned to the
// caller via a pointer argument.
//
// Input tokens are represented by a linked list. Unlike many recursive
// descent parsers, we don't have the notion of the "input token stream".
// Most parsing functions don't change the global state of the parser.
// So it is very easy to lookahead arbitrary number of tokens in this
// parser.

#include "chibicc.h"

// All local variable instances created during parsing are
// accumulated to this list.
Obj *locals;

static Node *compound_stmt(Token **rest, Token *tok);
static Node *stmt(Token **rest, Token *tok);
static Node *expr_stmt(Token **rest, Token *tok);
static Node *expr(Token **rest, Token *tok);
static Node *assign(Token **rest, Token *tok);
static Node *equality(Token **rest, Token *tok);
static Node *relational(Token **rest, Token *tok);
static Node *add(Token **rest, Token *tok);
static Node *mul(Token **rest, Token *tok);
static Node *unary(Token **rest, Token *tok);
static Node *primary(Token **rest, Token *tok);

// Find a local variable by name.
static Obj *find_var(Token *tok) {
  for (Obj *var = locals; var; var = var->next)
    if (strlen(var->name) == tok->len && !strncmp(tok->loc, var->name, tok->len))
      return var;
  return NULL;
}

static Node *new_node(NodeKind kind, Token *tok) {
  Node *node = calloc(1, sizeof(Node));
  node->kind = kind;
  node->tok = tok;
  return node;
}

static Node *new_binary(NodeKind kind, Node *lhs, Node *rhs, Token *tok) {
  Node *node = new_node(kind, tok);
  node->lhs = lhs;
  node->rhs = rhs;
  return node;
}

static Node *new_unary(NodeKind kind, Node *expr, Token *tok) {
  Node *node = new_node(kind, tok);
  node->lhs = expr;
  return node;
}

static Node *new_num(int val, Token *tok) {
  Node *node = new_node(ND_NUM, tok);
  node->val = val;
  return node;
}

static Node *new_var_node(Obj *var, Token *tok) {
  Node *node = new_node(ND_VAR, tok);
  node->var = var;
  return node;
}

static Obj *new_lvar(char *name) {
  Obj *var = calloc(1, sizeof(Obj));
  var->name = name;
  var->next = locals;
  locals = var;
  return var;
}

// stmt = "return" expr ";"
//      | "if" "(" expr ")" stmt ("else" stmt)?
//      | "for" "(" expr-stmt expr? ";" expr? ")" stmt
//      | "while" "(" expr ")" stmt
//      | "{" compound-stmt
//      | expr-stmt
static Node *stmt(Token **rest, Token *tok) {
  if (equal(tok, "return")) {
    Node *node = new_node(ND_RETURN, tok);
    node->lhs = expr(&tok, tok->next);
    *rest = skip(tok, ";");
    return node;
  }

  if (equal(tok, "if")) {
    Node *node = new_node(ND_IF, tok);
    tok = skip(tok->next, "(");
    node->cond = expr(&tok, tok);
    tok = skip(tok, ")");
    node->then = stmt(&tok, tok);
    if (equal(tok, "else"))
      node->els = stmt(&tok, tok->next);
    *rest = tok;
    return node;
  }

  if (equal(tok, "for")) {
    Node *node = new_node(ND_FOR, tok);
    tok = skip(tok->next, "(");

    node->init = expr_stmt(&tok, tok);

    if (!equal(tok, ";"))
      node->cond = expr(&tok, tok);
    tok = skip(tok, ";");

    if (!equal(tok, ")"))
      node->inc = expr(&tok, tok);
    tok = skip(tok, ")");

    node->then = stmt(rest, tok);
    return node;
  }

  if (equal(tok, "while")) {
    Node *node = new_node(ND_FOR, tok);
    tok = skip(tok->next, "(");
    node->cond = expr(&tok, tok);
    tok = skip(tok, ")");
    node->then = stmt(rest, tok);
    return node;
  }

  if (equal(tok, "{"))
    return compound_stmt(rest, tok->next);

  return expr_stmt(rest, tok);
}

// compound-stmt = stmt* "}"
static Node *compound_stmt(Token **rest, Token *tok) {
  Node *node = new_node(ND_BLOCK, tok);

  Node head = {};
  Node *cur = &head;
  while (!equal(tok, "}")) {
    cur = cur->next = stmt(&tok, tok);
    add_type(cur);
  }

  node->body = head.next;
  *rest = tok->next;
  return node;
}

// expr-stmt = expr? ";"
static Node *expr_stmt(Token **rest, Token *tok) {
  if (equal(tok, ";")) {
    *rest = tok->next;
    return new_node(ND_BLOCK, tok);
  }

  Node *node = new_node(ND_EXPR_STMT, tok);
  node->lhs = expr(&tok, tok);
  *rest = skip(tok, ";");
  return node;
}

// expr = assign
static Node *expr(Token **rest, Token *tok) {
  return assign(rest, tok);
}

// assign = equality ("=" assign)?
static Node *assign(Token **rest, Token *tok) {
  Node *node = equality(&tok, tok);

  if (equal(tok, "="))
    return new_binary(ND_ASSIGN, node, assign(rest, tok->next), tok);

  *rest = tok;
  return node;
}

// equality = relational ("==" relational | "!=" relational)*
static Node *equality(Token **rest, Token *tok) {
  Node *node = relational(&tok, tok);

  for (;;) {
    Token *start = tok;

    if (equal(tok, "==")) {
      node = new_binary(ND_EQ, node, relational(&tok, tok->next), start);
      continue;
    }

    if (equal(tok, "!=")) {
      node = new_binary(ND_NE, node, relational(&tok, tok->next), start);
      continue;
    }

    *rest = tok;
    return node;
  }
}

// relational = add ("<" add | "<=" add | ">" add | ">=" add)*
static Node *relational(Token **rest, Token *tok) {
  Node *node = add(&tok, tok);

  for (;;) {
    Token *start = tok;

    if (equal(tok, "<")) {
      node = new_binary(ND_LT, node, add(&tok, tok->next), start);
      continue;
    }

    if (equal(tok, "<=")) {
      node = new_binary(ND_LE, node, add(&tok, tok->next), start);
      continue;
    }

    if (equal(tok, ">")) {
      node = new_binary(ND_LT, add(&tok, tok->next), node, start);
      continue;
    }

    if (equal(tok, ">=")) {
      node = new_binary(ND_LE, add(&tok, tok->next), node, start);
      continue;
    }

    *rest = tok;
    return node;
  }
}

// In C, `+` operator is overloaded to perform the pointer arithmetic.
// If p is a pointer, p+n adds not n but sizeof(*p)*n to the value of p,
// so that p+n points to the location n elements (not bytes) ahead of p.
// In other words, we need to scale an integer value before adding to a
// pointer value. This function takes care of the scaling.
static Node *new_add(Node *lhs, Node *rhs, Token *tok) {
  add_type(lhs);
  add_type(rhs);

  // num + num
  if (is_integer(lhs->ty) && is_integer(rhs->ty))
    return new_binary(ND_ADD, lhs, rhs, tok);

  if (lhs->ty->base && rhs->ty->base)
    error_tok(tok, "invalid operands");

  // Canonicalize `num + ptr` to `ptr + num`.
  if (!lhs->ty->base && rhs->ty->base) {
    Node *tmp = lhs;
    lhs = rhs;
    rhs = tmp;
  }

  // ptr + num
  rhs = new_binary(ND_MUL, rhs, new_num(8, tok), tok);
  return new_binary(ND_ADD, lhs, rhs, tok);
}

// Like `+`, `-` is overloaded for the pointer type.
static Node *new_sub(Node *lhs, Node *rhs, Token *tok) {
  add_type(lhs);
  add_type(rhs);

  // num - num
  if (is_integer(lhs->ty) && is_integer(rhs->ty))
    return new_binary(ND_SUB, lhs, rhs, tok);

  // ptr - num
  if (lhs->ty->base && is_integer(rhs->ty)) {
    rhs = new_binary(ND_MUL, rhs, new_num(8, tok), tok);
    add_type(rhs);
    Node *node = new_binary(ND_SUB, lhs, rhs, tok);
    node->ty = lhs->ty;
    return node;
  }

  // ptr - ptr, which returns how many elements are between the two.
  if (lhs->ty->base && rhs->ty->base) {
    Node *node = new_binary(ND_SUB, lhs, rhs, tok);
    node->ty = ty_int;
    return new_binary(ND_DIV, node, new_num(8, tok), tok);
  }

  error_tok(tok, "invalid operands");
}

// add = mul ("+" mul | "-" mul)*
static Node *add(Token **rest, Token *tok) {
  Node *node = mul(&tok, tok);

  for (;;) {
    Token *start = tok;

    if (equal(tok, "+")) {
      node = new_add(node, mul(&tok, tok->next), start);
      continue;
    }

    if (equal(tok, "-")) {
      node = new_sub(node, mul(&tok, tok->next), start);
      continue;
    }

    *rest = tok;
    return node;
  }
}

// mul = unary ("*" unary | "/" unary)*
static Node *mul(Token **rest, Token *tok) {
  Node *node = unary(&tok, tok);

  for (;;) {
    Token *start = tok;

    if (equal(tok, "*")) {
      node = new_binary(ND_MUL, node, unary(&tok, tok->next), start);
      continue;
    }

    if (equal(tok, "/")) {
      node = new_binary(ND_DIV, node, unary(&tok, tok->next), start);
      continue;
    }

    *rest = tok;
    return node;
  }
}

// unary = ("+" | "-" | "*" | "&") unary
//       | primary
static Node *unary(Token **rest, Token *tok) {
  if (equal(tok, "+"))
    return unary(rest, tok->next);

  if (equal(tok, "-"))
    return new_unary(ND_NEG, unary(rest, tok->next), tok);

  if (equal(tok, "&"))
    return new_unary(ND_ADDR, unary(rest, tok->next), tok);

  if (equal(tok, "*"))
    return new_unary(ND_DEREF, unary(rest, tok->next), tok);

  return primary(rest, tok);
}

// primary = "(" expr ")" | ident | num
static Node *primary(Token **rest, Token *tok) {
  if (equal(tok, "(")) {
    Node *node = expr(&tok, tok->next);
    *rest = skip(tok, ")");
    return node;
  }

  if (tok->kind == TK_IDENT) {
    Obj *var = find_var(tok);
    if (!var)
      var = new_lvar(strndup(tok->loc, tok->len));
    *rest = tok->next;
    return new_var_node(var, tok);
  }

  if (tok->kind == TK_NUM) {
    Node *node = new_num(tok->val, tok);
    *rest = tok->next;
    return node;
  }

  error_tok(tok, "expected an expression");
}

// program = stmt*
Function *parse(Token *tok) {
  tok = skip(tok, "{");

  Function *prog = calloc(1, sizeof(Function));
  prog->body = compound_stmt(&tok, tok);
  prog->locals = locals;
  return prog;
}