Initial public snapshot

With code from Bill. Thanks Bill!

1 files changed, 1208 insertions, 0 deletions

diff --git a/src/3p/chibicc/preprocess.c b/src/3p/chibicc/preprocess.c
new file mode 100644
index 0000000..cd8d1d8
--- /dev/null
+++ b/src/3p/chibicc/preprocess.c
@@ -0,0 +1,1208 @@
+// This file implements the C preprocessor.
+//
+// The preprocessor takes a list of tokens as an input and returns a
+// new list of tokens as an output.
+//
+// The preprocessing language is designed in such a way that that's
+// guaranteed to stop even if there is a recursive macro.
+// Informally speaking, a macro is applied only once for each token.
+// That is, if a macro token T appears in a result of direct or
+// indirect macro expansion of T, T won't be expanded any further.
+// For example, if T is defined as U, and U is defined as T, then
+// token T is expanded to U and then to T and the macro expansion
+// stops at that point.
+//
+// To achieve the above behavior, we attach for each token a set of
+// macro names from which the token is expanded. The set is called
+// "hideset". Hideset is initially empty, and every time we expand a
+// macro, the macro name is added to the resulting tokens' hidesets.
+//
+// The above macro expansion algorithm is explained in this document
+// written by Dave Prossor, which is used as a basis for the
+// standard's wording:
+// https://github.com/rui314/chibicc/wiki/cpp.algo.pdf
+
+#include "chibicc.h"
+
+typedef struct MacroParam MacroParam;
+struct MacroParam {
+  MacroParam *next;
+  char *name;
+};
+
+typedef struct MacroArg MacroArg;
+struct MacroArg {
+  MacroArg *next;
+  char *name;
+  bool is_va_args;
+  Token *tok;
+};
+
+typedef Token *macro_handler_fn(Token *);
+
+typedef struct Macro Macro;
+struct Macro {
+  char *name;
+  bool is_objlike; // Object-like or function-like
+  MacroParam *params;
+  char *va_args_name;
+  Token *body;
+  macro_handler_fn *handler;
+};
+
+// `#if` can be nested, so we use a stack to manage nested `#if`s.
+typedef struct CondIncl CondIncl;
+struct CondIncl {
+  CondIncl *next;
+  enum { IN_THEN, IN_ELIF, IN_ELSE } ctx;
+  Token *tok;
+  bool included;
+};
+
+typedef struct Hideset Hideset;
+struct Hideset {
+  Hideset *next;
+  char *name;
+};
+
+static HashMap macros;
+static CondIncl *cond_incl;
+static HashMap pragma_once;
+static int include_next_idx;
+
+static Token *preprocess2(Token *tok);
+static Macro *find_macro(Token *tok);
+
+static bool is_hash(Token *tok) {
+  return tok->at_bol && equal(tok, "#");
+}
+
+// Some preprocessor directives such as #include allow extraneous
+// tokens before newline. This function skips such tokens.
+static Token *skip_line(Token *tok) {
+  if (tok->at_bol)
+    return tok;
+  warn_tok(tok, "extra token");
+  while (!tok->at_bol)
+    tok = tok->next;
+  return tok;
+}
+
+static Token *copy_token(Token *tok) {
+  Token *t = calloc(1, sizeof(Token));
+  *t = *tok;
+  t->next = NULL;
+  return t;
+}
+
+static Token *new_eof(Token *tok) {
+  Token *t = copy_token(tok);
+  t->kind = TK_EOF;
+  t->len = 0;
+  return t;
+}
+
+static Hideset *new_hideset(char *name) {
+  Hideset *hs = calloc(1, sizeof(Hideset));
+  hs->name = name;
+  return hs;
+}
+
+static Hideset *hideset_union(Hideset *hs1, Hideset *hs2) {
+  Hideset head = {};
+  Hideset *cur = &head;
+
+  for (; hs1; hs1 = hs1->next)
+    cur = cur->next = new_hideset(hs1->name);
+  cur->next = hs2;
+  return head.next;
+}
+
+static bool hideset_contains(Hideset *hs, char *s, int len) {
+  for (; hs; hs = hs->next)
+    if (strlen(hs->name) == len && !strncmp(hs->name, s, len))
+      return true;
+  return false;
+}
+
+static Hideset *hideset_intersection(Hideset *hs1, Hideset *hs2) {
+  Hideset head = {};
+  Hideset *cur = &head;
+
+  for (; hs1; hs1 = hs1->next)
+    if (hideset_contains(hs2, hs1->name, strlen(hs1->name)))
+      cur = cur->next = new_hideset(hs1->name);
+  return head.next;
+}
+
+static Token *add_hideset(Token *tok, Hideset *hs) {
+  Token head = {};
+  Token *cur = &head;
+
+  for (; tok; tok = tok->next) {
+    Token *t = copy_token(tok);
+    t->hideset = hideset_union(t->hideset, hs);
+    cur = cur->next = t;
+  }
+  return head.next;
+}
+
+// Append tok2 to the end of tok1.
+static Token *append(Token *tok1, Token *tok2) {
+  if (tok1->kind == TK_EOF)
+    return tok2;
+
+  Token head = {};
+  Token *cur = &head;
+
+  for (; tok1->kind != TK_EOF; tok1 = tok1->next)
+    cur = cur->next = copy_token(tok1);
+  cur->next = tok2;
+  return head.next;
+}
+
+static Token *skip_cond_incl2(Token *tok) {
+  while (tok->kind != TK_EOF) {
+    if (is_hash(tok) &&
+        (equal(tok->next, "if") || equal(tok->next, "ifdef") ||
+         equal(tok->next, "ifndef"))) {
+      tok = skip_cond_incl2(tok->next->next);
+      continue;
+    }
+    if (is_hash(tok) && equal(tok->next, "endif"))
+      return tok->next->next;
+    tok = tok->next;
+  }
+  return tok;
+}
+
+// Skip until next `#else`, `#elif` or `#endif`.
+// Nested `#if` and `#endif` are skipped.
+static Token *skip_cond_incl(Token *tok) {
+  while (tok->kind != TK_EOF) {
+    if (is_hash(tok) &&
+        (equal(tok->next, "if") || equal(tok->next, "ifdef") ||
+         equal(tok->next, "ifndef"))) {
+      tok = skip_cond_incl2(tok->next->next);
+      continue;
+    }
+
+    if (is_hash(tok) &&
+        (equal(tok->next, "elif") || equal(tok->next, "else") ||
+         equal(tok->next, "endif")))
+      break;
+    tok = tok->next;
+  }
+  return tok;
+}
+
+// Double-quote a given string and returns it.
+static char *quote_string(char *str) {
+  int bufsize = 3;
+  for (int i = 0; str[i]; i++) {
+    if (str[i] == '\\' || str[i] == '"')
+      bufsize++;
+    bufsize++;
+  }
+
+  char *buf = calloc(1, bufsize);
+  char *p = buf;
+  *p++ = '"';
+  for (int i = 0; str[i]; i++) {
+    if (str[i] == '\\' || str[i] == '"')
+      *p++ = '\\';
+    *p++ = str[i];
+  }
+  *p++ = '"';
+  *p++ = '\0';
+  return buf;
+}
+
+static Token *new_str_token(char *str, Token *tmpl) {
+  char *buf = quote_string(str);
+  return tokenize(new_file(tmpl->file->name, tmpl->file->file_no, buf));
+}
+
+// Copy all tokens until the next newline, terminate them with
+// an EOF token and then returns them. This function is used to
+// create a new list of tokens for `#if` arguments.
+static Token *copy_line(Token **rest, Token *tok) {
+  Token head = {};
+  Token *cur = &head;
+
+  for (; !tok->at_bol; tok = tok->next)
+    cur = cur->next = copy_token(tok);
+
+  cur->next = new_eof(tok);
+  *rest = tok;
+  return head.next;
+}
+
+static Token *new_num_token(int val, Token *tmpl) {
+  char *buf = format("%d\n", val);
+  return tokenize(new_file(tmpl->file->name, tmpl->file->file_no, buf));
+}
+
+static Token *read_const_expr(Token **rest, Token *tok) {
+  tok = copy_line(rest, tok);
+
+  Token head = {};
+  Token *cur = &head;
+
+  while (tok->kind != TK_EOF) {
+    // "defined(foo)" or "defined foo" becomes "1" if macro "foo"
+    // is defined. Otherwise "0".
+    if (equal(tok, "defined")) {
+      Token *start = tok;
+      bool has_paren = consume(&tok, tok->next, "(");
+
+      if (tok->kind != TK_IDENT)
+        error_tok(start, "macro name must be an identifier");
+      Macro *m = find_macro(tok);
+      tok = tok->next;
+
+      if (has_paren)
+        tok = skip(tok, ")");
+
+      cur = cur->next = new_num_token(m ? 1 : 0, start);
+      continue;
+    }
+
+    cur = cur->next = tok;
+    tok = tok->next;
+  }
+
+  cur->next = tok;
+  return head.next;
+}
+
+// Read and evaluate a constant expression.
+static long eval_const_expr(Token **rest, Token *tok) {
+  Token *start = tok;
+  Token *expr = read_const_expr(rest, tok->next);
+  expr = preprocess2(expr);
+
+  if (expr->kind == TK_EOF)
+    error_tok(start, "no expression");
+
+  // [https://www.sigbus.info/n1570#6.10.1p4] The standard requires
+  // we replace remaining non-macro identifiers with "0" before
+  // evaluating a constant expression. For example, `#if foo` is
+  // equivalent to `#if 0` if foo is not defined.
+  for (Token *t = expr; t->kind != TK_EOF; t = t->next) {
+    if (t->kind == TK_IDENT) {
+      Token *next = t->next;
+      *t = *new_num_token(0, t);
+      t->next = next;
+    }
+  }
+
+  // Convert pp-numbers to regular numbers
+  convert_pp_tokens(expr);
+
+  Token *rest2;
+  long val = const_expr(&rest2, expr);
+  if (rest2->kind != TK_EOF)
+    error_tok(rest2, "extra token");
+  return val;
+}
+
+static CondIncl *push_cond_incl(Token *tok, bool included) {
+  CondIncl *ci = calloc(1, sizeof(CondIncl));
+  ci->next = cond_incl;
+  ci->ctx = IN_THEN;
+  ci->tok = tok;
+  ci->included = included;
+  cond_incl = ci;
+  return ci;
+}
+
+static Macro *find_macro(Token *tok) {
+  if (tok->kind != TK_IDENT)
+    return NULL;
+  return hashmap_get2(&macros, tok->loc, tok->len);
+}
+
+static Macro *add_macro(char *name, bool is_objlike, Token *body) {
+  Macro *m = calloc(1, sizeof(Macro));
+  m->name = name;
+  m->is_objlike = is_objlike;
+  m->body = body;
+  hashmap_put(&macros, name, m);
+  return m;
+}
+
+static MacroParam *read_macro_params(Token **rest, Token *tok, char **va_args_name) {
+  MacroParam head = {};
+  MacroParam *cur = &head;
+
+  while (!equal(tok, ")")) {
+    if (cur != &head)
+      tok = skip(tok, ",");
+
+    if (equal(tok, "...")) {
+      *va_args_name = "__VA_ARGS__";
+      *rest = skip(tok->next, ")");
+      return head.next;
+    }
+
+    if (tok->kind != TK_IDENT)
+      error_tok(tok, "expected an identifier");
+
+    if (equal(tok->next, "...")) {
+      *va_args_name = strndup(tok->loc, tok->len);
+      *rest = skip(tok->next->next, ")");
+      return head.next;
+    }
+
+    MacroParam *m = calloc(1, sizeof(MacroParam));
+    m->name = strndup(tok->loc, tok->len);
+    cur = cur->next = m;
+    tok = tok->next;
+  }
+
+  *rest = tok->next;
+  return head.next;
+}
+
+static void read_macro_definition(Token **rest, Token *tok) {
+  if (tok->kind != TK_IDENT)
+    error_tok(tok, "macro name must be an identifier");
+  char *name = strndup(tok->loc, tok->len);
+  tok = tok->next;
+
+  if (!tok->has_space && equal(tok, "(")) {
+    // Function-like macro
+    char *va_args_name = NULL;
+    MacroParam *params = read_macro_params(&tok, tok->next, &va_args_name);
+
+    Macro *m = add_macro(name, false, copy_line(rest, tok));
+    m->params = params;
+    m->va_args_name = va_args_name;
+  } else {
+    // Object-like macro
+    add_macro(name, true, copy_line(rest, tok));
+  }
+}
+
+static MacroArg *read_macro_arg_one(Token **rest, Token *tok, bool read_rest) {
+  Token head = {};
+  Token *cur = &head;
+  int level = 0;
+
+  for (;;) {
+    if (level == 0 && equal(tok, ")"))
+      break;
+    if (level == 0 && !read_rest && equal(tok, ","))
+      break;
+
+    if (tok->kind == TK_EOF)
+      error_tok(tok, "premature end of input");
+
+    if (equal(tok, "("))
+      level++;
+    else if (equal(tok, ")"))
+      level--;
+
+    cur = cur->next = copy_token(tok);
+    tok = tok->next;
+  }
+
+  cur->next = new_eof(tok);
+
+  MacroArg *arg = calloc(1, sizeof(MacroArg));
+  arg->tok = head.next;
+  *rest = tok;
+  return arg;
+}
+
+static MacroArg *
+read_macro_args(Token **rest, Token *tok, MacroParam *params, char *va_args_name) {
+  Token *start = tok;
+  tok = tok->next->next;
+
+  MacroArg head = {};
+  MacroArg *cur = &head;
+
+  MacroParam *pp = params;
+  for (; pp; pp = pp->next) {
+    if (cur != &head)
+      tok = skip(tok, ",");
+    cur = cur->next = read_macro_arg_one(&tok, tok, false);
+    cur->name = pp->name;
+  }
+
+  if (va_args_name) {
+    MacroArg *arg;
+    if (equal(tok, ")")) {
+      arg = calloc(1, sizeof(MacroArg));
+      arg->tok = new_eof(tok);
+    } else {
+      if (pp != params)
+        tok = skip(tok, ",");
+      arg = read_macro_arg_one(&tok, tok, true);
+    }
+    arg->name = va_args_name;;
+    arg->is_va_args = true;
+    cur = cur->next = arg;
+  } else if (pp) {
+    error_tok(start, "too many arguments");
+  }
+
+  skip(tok, ")");
+  *rest = tok;
+  return head.next;
+}
+
+static MacroArg *find_arg(MacroArg *args, Token *tok) {
+  for (MacroArg *ap = args; ap; ap = ap->next)
+    if (tok->len == strlen(ap->name) && !strncmp(tok->loc, ap->name, tok->len))
+      return ap;
+  return NULL;
+}
+
+// Concatenates all tokens in `tok` and returns a new string.
+static char *join_tokens(Token *tok, Token *end) {
+  // Compute the length of the resulting token.
+  int len = 1;
+  for (Token *t = tok; t != end && t->kind != TK_EOF; t = t->next) {
+    if (t != tok && t->has_space)
+      len++;
+    len += t->len;
+  }
+
+  char *buf = calloc(1, len);
+
+  // Copy token texts.
+  int pos = 0;
+  for (Token *t = tok; t != end && t->kind != TK_EOF; t = t->next) {
+    if (t != tok && t->has_space)
+      buf[pos++] = ' ';
+    strncpy(buf + pos, t->loc, t->len);
+    pos += t->len;
+  }
+  buf[pos] = '\0';
+  return buf;
+}
+
+// Concatenates all tokens in `arg` and returns a new string token.
+// This function is used for the stringizing operator (#).
+static Token *stringize(Token *hash, Token *arg) {
+  // Create a new string token. We need to set some value to its
+  // source location for error reporting function, so we use a macro
+  // name token as a template.
+  char *s = join_tokens(arg, NULL);
+  return new_str_token(s, hash);
+}
+
+// Concatenate two tokens to create a new token.
+static Token *paste(Token *lhs, Token *rhs) {
+  // Paste the two tokens.
+  char *buf = format("%.*s%.*s", lhs->len, lhs->loc, rhs->len, rhs->loc);
+
+  // Tokenize the resulting string.
+  Token *tok = tokenize(new_file(lhs->file->name, lhs->file->file_no, buf));
+  if (tok->next->kind != TK_EOF)
+    error_tok(lhs, "pasting forms '%s', an invalid token", buf);
+  return tok;
+}
+
+static bool has_varargs(MacroArg *args) {
+  for (MacroArg *ap = args; ap; ap = ap->next)
+    if (!strcmp(ap->name, "__VA_ARGS__"))
+      return ap->tok->kind != TK_EOF;
+  return false;
+}
+
+// Replace func-like macro parameters with given arguments.
+static Token *subst(Token *tok, MacroArg *args) {
+  Token head = {};
+  Token *cur = &head;
+
+  while (tok->kind != TK_EOF) {
+    // "#" followed by a parameter is replaced with stringized actuals.
+    if (equal(tok, "#")) {
+      MacroArg *arg = find_arg(args, tok->next);
+      if (!arg)
+        error_tok(tok->next, "'#' is not followed by a macro parameter");
+      cur = cur->next = stringize(tok, arg->tok);
+      tok = tok->next->next;
+      continue;
+    }
+
+    // [GNU] If __VA_ARG__ is empty, `,##__VA_ARGS__` is expanded
+    // to the empty token list. Otherwise, its expaned to `,` and
+    // __VA_ARGS__.
+    if (equal(tok, ",") && equal(tok->next, "##")) {
+      MacroArg *arg = find_arg(args, tok->next->next);
+      if (arg && arg->is_va_args) {
+        if (arg->tok->kind == TK_EOF) {
+          tok = tok->next->next->next;
+        } else {
+          cur = cur->next = copy_token(tok);
+          tok = tok->next->next;
+        }
+        continue;
+      }
+    }
+
+    if (equal(tok, "##")) {
+      if (cur == &head)
+        error_tok(tok, "'##' cannot appear at start of macro expansion");
+
+      if (tok->next->kind == TK_EOF)
+        error_tok(tok, "'##' cannot appear at end of macro expansion");
+
+      MacroArg *arg = find_arg(args, tok->next);
+      if (arg) {
+        if (arg->tok->kind != TK_EOF) {
+          *cur = *paste(cur, arg->tok);
+          for (Token *t = arg->tok->next; t->kind != TK_EOF; t = t->next)
+            cur = cur->next = copy_token(t);
+        }
+        tok = tok->next->next;
+        continue;
+      }
+
+      *cur = *paste(cur, tok->next);
+      tok = tok->next->next;
+      continue;
+    }
+
+    MacroArg *arg = find_arg(args, tok);
+
+    if (arg && equal(tok->next, "##")) {
+      Token *rhs = tok->next->next;
+
+      if (arg->tok->kind == TK_EOF) {
+        MacroArg *arg2 = find_arg(args, rhs);
+        if (arg2) {
+          for (Token *t = arg2->tok; t->kind != TK_EOF; t = t->next)
+            cur = cur->next = copy_token(t);
+        } else {
+          cur = cur->next = copy_token(rhs);
+        }
+        tok = rhs->next;
+        continue;
+      }
+
+      for (Token *t = arg->tok; t->kind != TK_EOF; t = t->next)
+        cur = cur->next = copy_token(t);
+      tok = tok->next;
+      continue;
+    }
+
+    // If __VA_ARG__ is empty, __VA_OPT__(x) is expanded to the
+    // empty token list. Otherwise, __VA_OPT__(x) is expanded to x.
+    if (equal(tok, "__VA_OPT__") && equal(tok->next, "(")) {
+      MacroArg *arg = read_macro_arg_one(&tok, tok->next->next, true);
+      if (has_varargs(args))
+        for (Token *t = arg->tok; t->kind != TK_EOF; t = t->next)
+          cur = cur->next = t;
+      tok = skip(tok, ")");
+      continue;
+    }
+
+    // Handle a macro token. Macro arguments are completely macro-expanded
+    // before they are substituted into a macro body.
+    if (arg) {
+      Token *t = preprocess2(arg->tok);
+      t->at_bol = tok->at_bol;
+      t->has_space = tok->has_space;
+      for (; t->kind != TK_EOF; t = t->next)
+        cur = cur->next = copy_token(t);
+      tok = tok->next;
+      continue;
+    }
+
+    // Handle a non-macro token.
+    cur = cur->next = copy_token(tok);
+    tok = tok->next;
+    continue;
+  }
+
+  cur->next = tok;
+  return head.next;
+}
+
+// If tok is a macro, expand it and return true.
+// Otherwise, do nothing and return false.
+static bool expand_macro(Token **rest, Token *tok) {
+  if (hideset_contains(tok->hideset, tok->loc, tok->len))
+    return false;
+
+  Macro *m = find_macro(tok);
+  if (!m)
+    return false;
+
+  // Built-in dynamic macro application such as __LINE__
+  if (m->handler) {
+    *rest = m->handler(tok);
+    (*rest)->next = tok->next;
+    return true;
+  }
+
+  // Object-like macro application
+  if (m->is_objlike) {
+    Hideset *hs = hideset_union(tok->hideset, new_hideset(m->name));
+    Token *body = add_hideset(m->body, hs);
+    for (Token *t = body; t->kind != TK_EOF; t = t->next)
+      t->origin = tok;
+    *rest = append(body, tok->next);
+    (*rest)->at_bol = tok->at_bol;
+    (*rest)->has_space = tok->has_space;
+    return true;
+  }
+
+  // If a funclike macro token is not followed by an argument list,
+  // treat it as a normal identifier.
+  if (!equal(tok->next, "("))
+    return false;
+
+  // Function-like macro application
+  Token *macro_token = tok;
+  MacroArg *args = read_macro_args(&tok, tok, m->params, m->va_args_name);
+  Token *rparen = tok;
+
+  // Tokens that consist a func-like macro invocation may have different
+  // hidesets, and if that's the case, it's not clear what the hideset
+  // for the new tokens should be. We take the interesection of the
+  // macro token and the closing parenthesis and use it as a new hideset
+  // as explained in the Dave Prossor's algorithm.
+  Hideset *hs = hideset_intersection(macro_token->hideset, rparen->hideset);
+  hs = hideset_union(hs, new_hideset(m->name));
+
+  Token *body = subst(m->body, args);
+  body = add_hideset(body, hs);
+  for (Token *t = body; t->kind != TK_EOF; t = t->next)
+    t->origin = macro_token;
+  *rest = append(body, tok->next);
+  (*rest)->at_bol = macro_token->at_bol;
+  (*rest)->has_space = macro_token->has_space;
+  return true;
+}
+
+char *search_include_paths(char *filename) {
+  if (filename[0] == '/')
+    return filename;
+
+  static HashMap cache;
+  char *cached = hashmap_get(&cache, filename);
+  if (cached)
+    return cached;
+
+  // Search a file from the include paths.
+  for (int i = 0; i < include_paths.len; i++) {
+    char *path = format("%s/%s", include_paths.data[i], filename);
+    if (!file_exists(path))
+      continue;
+    hashmap_put(&cache, filename, path);
+    include_next_idx = i + 1;
+    return path;
+  }
+  return NULL;
+}
+
+static char *search_include_next(char *filename) {
+  for (; include_next_idx < include_paths.len; include_next_idx++) {
+    char *path = format("%s/%s", include_paths.data[include_next_idx], filename);
+    if (file_exists(path))
+      return path;
+  }
+  return NULL;
+}
+
+// Read an #include argument.
+static char *read_include_filename(Token **rest, Token *tok, bool *is_dquote) {
+  // Pattern 1: #include "foo.h"
+  if (tok->kind == TK_STR) {
+    // A double-quoted filename for #include is a special kind of
+    // token, and we don't want to interpret any escape sequences in it.
+    // For example, "\f" in "C:\foo" is not a formfeed character but
+    // just two non-control characters, backslash and f.
+    // So we don't want to use token->str.
+    *is_dquote = true;
+    *rest = skip_line(tok->next);
+    return strndup(tok->loc + 1, tok->len - 2);
+  }
+
+  // Pattern 2: #include <foo.h>
+  if (equal(tok, "<")) {
+    // Reconstruct a filename from a sequence of tokens between
+    // "<" and ">".
+    Token *start = tok;
+
+    // Find closing ">".
+    for (; !equal(tok, ">"); tok = tok->next)
+      if (tok->at_bol || tok->kind == TK_EOF)
+        error_tok(tok, "expected '>'");
+
+    *is_dquote = false;
+    *rest = skip_line(tok->next);
+    return join_tokens(start->next, tok);
+  }
+
+  // Pattern 3: #include FOO
+  // In this case FOO must be macro-expanded to either
+  // a single string token or a sequence of "<" ... ">".
+  if (tok->kind == TK_IDENT) {
+    Token *tok2 = preprocess2(copy_line(rest, tok));
+    return read_include_filename(&tok2, tok2, is_dquote);
+  }
+
+  error_tok(tok, "expected a filename");
+}
+
+// Detect the following "include guard" pattern.
+//
+//   #ifndef FOO_H
+//   #define FOO_H
+//   ...
+//   #endif
+static char *detect_include_guard(Token *tok) {
+  // Detect the first two lines.
+  if (!is_hash(tok) || !equal(tok->next, "ifndef"))
+    return NULL;
+  tok = tok->next->next;
+
+  if (tok->kind != TK_IDENT)
+    return NULL;
+
+  char *macro = strndup(tok->loc, tok->len);
+  tok = tok->next;
+
+  if (!is_hash(tok) || !equal(tok->next, "define") || !equal(tok->next->next, macro))
+    return NULL;
+
+  // Read until the end of the file.
+  while (tok->kind != TK_EOF) {
+    if (!is_hash(tok)) {
+      tok = tok->next;
+      continue;
+    }
+
+    if (equal(tok->next, "endif") && tok->next->next->kind == TK_EOF)
+      return macro;
+
+    if (equal(tok, "if") || equal(tok, "ifdef") || equal(tok, "ifndef"))
+      tok = skip_cond_incl(tok->next);
+    else
+      tok = tok->next;
+  }
+  return NULL;
+}
+
+static Token *include_file(Token *tok, char *path, Token *filename_tok) {
+  // Check for "#pragma once"
+  if (hashmap_get(&pragma_once, path))
+    return tok;
+
+  // If we read the same file before, and if the file was guarded
+  // by the usual #ifndef ... #endif pattern, we may be able to
+  // skip the file without opening it.
+  static HashMap include_guards;
+  char *guard_name = hashmap_get(&include_guards, path);
+  if (guard_name && hashmap_get(&macros, guard_name))
+    return tok;
+
+  Token *tok2 = tokenize_file(path);
+  if (!tok2)
+    error_tok(filename_tok, "%s: cannot open file: %s", path, strerror(errno));
+
+  guard_name = detect_include_guard(tok2);
+  if (guard_name)
+    hashmap_put(&include_guards, path, guard_name);
+
+  return append(tok2, tok);
+}
+
+// Read #line arguments
+static void read_line_marker(Token **rest, Token *tok) {
+  Token *start = tok;
+  tok = preprocess(copy_line(rest, tok));
+
+  if (tok->kind != TK_NUM || tok->ty->kind != TY_INT)
+    error_tok(tok, "invalid line marker");
+  start->file->line_delta = tok->val - start->line_no;
+
+  tok = tok->next;
+  if (tok->kind == TK_EOF)
+    return;
+
+  if (tok->kind != TK_STR)
+    error_tok(tok, "filename expected");
+  start->file->display_name = tok->str;
+}
+
+// Visit all tokens in `tok` while evaluating preprocessing
+// macros and directives.
+static Token *preprocess2(Token *tok) {
+  Token head = {};
+  Token *cur = &head;
+
+  while (tok->kind != TK_EOF) {
+    // If it is a macro, expand it.
+    if (expand_macro(&tok, tok))
+      continue;
+
+    // Pass through if it is not a "#".
+    if (!is_hash(tok)) {
+      tok->line_delta = tok->file->line_delta;
+      tok->filename = tok->file->display_name;
+      cur = cur->next = tok;
+      tok = tok->next;
+      continue;
+    }
+
+    Token *start = tok;
+    tok = tok->next;
+
+    if (equal(tok, "include")) {
+      bool is_dquote;
+      char *filename = read_include_filename(&tok, tok->next, &is_dquote);
+
+      if (filename[0] != '/' && is_dquote) {
+        char *path = format("%s/%s", dirname(strdup(start->file->name)), filename);
+        if (file_exists(path)) {
+          tok = include_file(tok, path, start->next->next);
+          continue;
+        }
+      }
+
+      char *path = search_include_paths(filename);
+      tok = include_file(tok, path ? path : filename, start->next->next);
+      continue;
+    }
+
+    if (equal(tok, "include_next")) {
+      bool ignore;
+      char *filename = read_include_filename(&tok, tok->next, &ignore);
+      char *path = search_include_next(filename);
+      tok = include_file(tok, path ? path : filename, start->next->next);
+      continue;
+    }
+
+    if (equal(tok, "define")) {
+      read_macro_definition(&tok, tok->next);
+      continue;
+    }
+
+    if (equal(tok, "undef")) {
+      tok = tok->next;
+      if (tok->kind != TK_IDENT)
+        error_tok(tok, "macro name must be an identifier");
+      undef_macro(strndup(tok->loc, tok->len));
+      tok = skip_line(tok->next);
+      continue;
+    }
+
+    if (equal(tok, "if")) {
+      long val = eval_const_expr(&tok, tok);
+      push_cond_incl(start, val);
+      if (!val)
+        tok = skip_cond_incl(tok);
+      continue;
+    }
+
+    if (equal(tok, "ifdef")) {
+      bool defined = find_macro(tok->next);
+      push_cond_incl(tok, defined);
+      tok = skip_line(tok->next->next);
+      if (!defined)
+        tok = skip_cond_incl(tok);
+      continue;
+    }
+
+    if (equal(tok, "ifndef")) {
+      bool defined = find_macro(tok->next);
+      push_cond_incl(tok, !defined);
+      tok = skip_line(tok->next->next);
+      if (defined)
+        tok = skip_cond_incl(tok);
+      continue;
+    }
+
+    if (equal(tok, "elif")) {
+      if (!cond_incl || cond_incl->ctx == IN_ELSE)
+        error_tok(start, "stray #elif");
+      cond_incl->ctx = IN_ELIF;
+
+      if (!cond_incl->included && eval_const_expr(&tok, tok))
+        cond_incl->included = true;
+      else
+        tok = skip_cond_incl(tok);
+      continue;
+    }
+
+    if (equal(tok, "else")) {
+      if (!cond_incl || cond_incl->ctx == IN_ELSE)
+        error_tok(start, "stray #else");
+      cond_incl->ctx = IN_ELSE;
+      tok = skip_line(tok->next);
+
+      if (cond_incl->included)
+        tok = skip_cond_incl(tok);
+      continue;
+    }
+
+    if (equal(tok, "endif")) {
+      if (!cond_incl)
+        error_tok(start, "stray #endif");
+      cond_incl = cond_incl->next;
+      tok = skip_line(tok->next);
+      continue;
+    }
+
+    if (equal(tok, "line")) {
+      read_line_marker(&tok, tok->next);
+      continue;
+    }
+
+    if (tok->kind == TK_PP_NUM) {
+      read_line_marker(&tok, tok);
+      continue;
+    }
+
+    if (equal(tok, "pragma") && equal(tok->next, "once")) {
+      hashmap_put(&pragma_once, tok->file->name, (void *)1);
+      tok = skip_line(tok->next->next);
+      continue;
+    }
+
+    if (equal(tok, "pragma")) {
+      do {
+        tok = tok->next;
+      } while (!tok->at_bol);
+      continue;
+    }
+
+    if (equal(tok, "error"))
+      error_tok(tok, "error");
+
+    // `#`-only line is legal. It's called a null directive.
+    if (tok->at_bol)
+      continue;
+
+    error_tok(tok, "invalid preprocessor directive");
+  }
+
+  cur->next = tok;
+  return head.next;
+}
+
+void define_macro(char *name, char *buf) {
+  Token *tok = tokenize(new_file("<built-in>", 1, buf));
+  add_macro(name, true, tok);
+}
+
+void undef_macro(char *name) {
+  hashmap_delete(&macros, name);
+}
+
+static Macro *add_builtin(char *name, macro_handler_fn *fn) {
+  Macro *m = add_macro(name, true, NULL);
+  m->handler = fn;
+  return m;
+}
+
+static Token *file_macro(Token *tmpl) {
+  while (tmpl->origin)
+    tmpl = tmpl->origin;
+  return new_str_token(tmpl->file->display_name, tmpl);
+}
+
+static Token *line_macro(Token *tmpl) {
+  while (tmpl->origin)
+    tmpl = tmpl->origin;
+  int i = tmpl->line_no + tmpl->file->line_delta;
+  return new_num_token(i, tmpl);
+}
+
+// __COUNTER__ is expanded to serial values starting from 0.
+static Token *counter_macro(Token *tmpl) {
+  static int i = 0;
+  return new_num_token(i++, tmpl);
+}
+
+// __TIMESTAMP__ is expanded to a string describing the last
+// modification time of the current file. E.g.
+// "Fri Jul 24 01:32:50 2020"
+static Token *timestamp_macro(Token *tmpl) {
+  struct stat st;
+  if (stat(tmpl->file->name, &st) != 0)
+    return new_str_token("??? ??? ?? ??:??:?? ????", tmpl);
+
+  char buf[30];
+  ctime_r(&st.st_mtime, buf);
+  buf[24] = '\0';
+  return new_str_token(buf, tmpl);
+}
+
+static Token *base_file_macro(Token *tmpl) {
+  return new_str_token(base_file, tmpl);
+}
+
+// __DATE__ is expanded to the current date, e.g. "May 17 2020".
+static char *format_date(struct tm *tm) {
+  static char mon[][4] = {
+    "Jan", "Feb", "Mar", "Apr", "May", "Jun",
+    "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
+  };
+
+  return format("\"%s %2d %d\"", mon[tm->tm_mon], tm->tm_mday, tm->tm_year + 1900);
+}
+
+// __TIME__ is expanded to the current time, e.g. "13:34:03".
+static char *format_time(struct tm *tm) {
+  return format("\"%02d:%02d:%02d\"", tm->tm_hour, tm->tm_min, tm->tm_sec);
+}
+
+void init_macros(void) {
+  // Define predefined macros
+  define_macro("_LP64", "1");
+  define_macro("__C99_MACRO_WITH_VA_ARGS", "1");
+  define_macro("__ELF__", "1");
+  define_macro("__LP64__", "1");
+  define_macro("__SIZEOF_DOUBLE__", "8");
+  define_macro("__SIZEOF_FLOAT__", "4");
+  define_macro("__SIZEOF_INT__", "4");
+  define_macro("__SIZEOF_LONG_DOUBLE__", "8");
+  define_macro("__SIZEOF_LONG_LONG__", "8");
+  define_macro("__SIZEOF_LONG__", "8");
+  define_macro("__SIZEOF_POINTER__", "8");
+  define_macro("__SIZEOF_PTRDIFF_T__", "8");
+  define_macro("__SIZEOF_SHORT__", "2");
+  define_macro("__SIZEOF_SIZE_T__", "8");
+  define_macro("__SIZE_TYPE__", "unsigned long");
+  define_macro("__STDC_HOSTED__", "1");
+  define_macro("__STDC_NO_COMPLEX__", "1");
+  define_macro("__STDC_UTF_16__", "1");
+  define_macro("__STDC_UTF_32__", "1");
+  define_macro("__STDC_VERSION__", "201112L");
+  define_macro("__STDC__", "1");
+  define_macro("__USER_LABEL_PREFIX__", "");
+  define_macro("__alignof__", "_Alignof");
+  define_macro("__amd64", "1");
+  define_macro("__amd64__", "1");
+  define_macro("__chibicc__", "1");
+  define_macro("__const__", "const");
+  define_macro("__gnu_linux__", "1");
+  define_macro("__inline__", "inline");
+  define_macro("__linux", "1");
+  define_macro("__linux__", "1");
+  define_macro("__signed__", "signed");
+  define_macro("__typeof__", "typeof");
+  define_macro("__unix", "1");
+  define_macro("__unix__", "1");
+  define_macro("__volatile__", "volatile");
+  define_macro("__x86_64", "1");
+  define_macro("__x86_64__", "1");
+  define_macro("linux", "1");
+  define_macro("unix", "1");
+
+  add_builtin("__FILE__", file_macro);
+  add_builtin("__LINE__", line_macro);
+  add_builtin("__COUNTER__", counter_macro);
+  add_builtin("__TIMESTAMP__", timestamp_macro);
+  add_builtin("__BASE_FILE__", base_file_macro);
+
+  time_t now = time(NULL);
+  struct tm *tm = localtime(&now);
+  define_macro("__DATE__", format_date(tm));
+  define_macro("__TIME__", format_time(tm));
+}
+
+typedef enum {
+  STR_NONE, STR_UTF8, STR_UTF16, STR_UTF32, STR_WIDE,
+} StringKind;
+
+static StringKind getStringKind(Token *tok) {
+  if (!strcmp(tok->loc, "u8"))
+    return STR_UTF8;
+
+  switch (tok->loc[0]) {
+  case '"': return STR_NONE;
+  case 'u': return STR_UTF16;
+  case 'U': return STR_UTF32;
+  case 'L': return STR_WIDE;
+  }
+  unreachable();
+}
+
+// Concatenate adjacent string literals into a single string literal
+// as per the C spec.
+static void join_adjacent_string_literals(Token *tok) {
+  // First pass: If regular string literals are adjacent to wide
+  // string literals, regular string literals are converted to a wide
+  // type before concatenation. In this pass, we do the conversion.
+  for (Token *tok1 = tok; tok1->kind != TK_EOF;) {
+    if (tok1->kind != TK_STR || tok1->next->kind != TK_STR) {
+      tok1 = tok1->next;
+      continue;
+    }
+
+    StringKind kind = getStringKind(tok1);
+    Type *basety = tok1->ty->base;
+
+    for (Token *t = tok1->next; t->kind == TK_STR; t = t->next) {
+      StringKind k = getStringKind(t);
+      if (kind == STR_NONE) {
+        kind = k;
+        basety = t->ty->base;
+      } else if (k != STR_NONE && kind != k) {
+        error_tok(t, "unsupported non-standard concatenation of string literals");
+      }
+    }
+
+    if (basety->size > 1)
+      for (Token *t = tok1; t->kind == TK_STR; t = t->next)
+        if (t->ty->base->size == 1)
+          *t = *tokenize_string_literal(t, basety);
+
+    while (tok1->kind == TK_STR)
+      tok1 = tok1->next;
+  }
+
+  // Second pass: concatenate adjacent string literals.
+  for (Token *tok1 = tok; tok1->kind != TK_EOF;) {
+    if (tok1->kind != TK_STR || tok1->next->kind != TK_STR) {
+      tok1 = tok1->next;
+      continue;
+    }
+
+    Token *tok2 = tok1->next;
+    while (tok2->kind == TK_STR)
+      tok2 = tok2->next;
+
+    int len = tok1->ty->array_len;
+    for (Token *t = tok1->next; t != tok2; t = t->next)
+      len = len + t->ty->array_len - 1;
+
+    char *buf = calloc(tok1->ty->base->size, len);
+
+    int i = 0;
+    for (Token *t = tok1; t != tok2; t = t->next) {
+      memcpy(buf + i, t->str, t->ty->size);
+      i = i + t->ty->size - t->ty->base->size;
+    }
+
+    *tok1 = *copy_token(tok1);
+    tok1->ty = array_of(tok1->ty->base, len);
+    tok1->str = buf;
+    tok1->next = tok2;
+    tok1 = tok2;
+  }
+}
+
+// Entry point function of the preprocessor.
+Token *preprocess(Token *tok) {
+  tok = preprocess2(tok);
+  if (cond_incl)
+    error_tok(cond_incl->tok, "unterminated conditional directive");
+  convert_pp_tokens(tok);
+  join_adjacent_string_literals(tok);
+
+  for (Token *t = tok; t; t = t->next)
+    t->line_no += t->line_delta;
+  return tok;
+}