[GNU] Add statement expression

This is a GNU C extension but will be useful for writing tests.

chibicc.h

@@ -98,6 +98,7 @@ typedef enum {
   ND_BLOCK,     // { ... }
   ND_FUNCALL,   // Function call
   ND_EXPR_STMT, // Expression statement
+  ND_STMT_EXPR, // Statement expression
   ND_VAR,       // Variable
   ND_NUM,       // Integer
 } NodeKind;
@@ -119,7 +120,7 @@ struct Node {
   Node *init;
   Node *inc;
 
-  // Block
+  // Block or statement expression
   Node *body;
 
   // Function call

codegen.c

@@ -6,6 +6,7 @@ static char *argreg64[] = {"%rdi", "%rsi", "%rdx", "%rcx", "%r8", "%r9"};
 static Obj *current_fn;
 
 static void gen_expr(Node *node);
+static void gen_stmt(Node *node);
 
 static int count(void) {
   static int i = 1;
@@ -104,6 +105,10 @@ static void gen_expr(Node *node) {
     gen_expr(node->rhs);
     store(node->ty);
     return;
+  case ND_STMT_EXPR:
+    for (Node *n = node->body; n; n = n->next)
+      gen_stmt(n);
+    return;
   case ND_FUNCALL: {
     int nargs = 0;
     for (Node *arg = node->args; arg; arg = arg->next) {

parse.c

@@ -545,8 +545,21 @@ static Node *funcall(Token **rest, Token *tok) {
   return node;
 }
 
-// primary = "(" expr ")" | "sizeof" unary | ident func-args? | str | num
+// primary = "(" "{" stmt+ "}" ")"
+//         | "(" expr ")"
+//         | "sizeof" unary
+//         | ident func-args?
+//         | str
+//         | num
 static Node *primary(Token **rest, Token *tok) {
+  if (equal(tok, "(") && equal(tok->next, "{")) {
+    // This is a GNU statement expresssion.
+    Node *node = new_node(ND_STMT_EXPR, tok);
+    node->body = compound_stmt(&tok, tok->next->next)->body;
+    *rest = skip(tok, ")");
+    return node;
+  }
+
   if (equal(tok, "(")) {
     Node *node = expr(&tok, tok->next);
     *rest = skip(tok, ")");

test.sh

@@ -207,4 +207,10 @@ assert 119 'int main() { return "\x77"[0]; }'
 assert 165 'int main() { return "\xA5"[0]; }'
 assert 255 'int main() { return "\x00ff"[0]; }'
 
+assert 0 'int main() { return ({ 0; }); }'
+assert 2 'int main() { return ({ 0; 1; 2; }); }'
+assert 1 'int main() { ({ 0; return 1; 2; }); return 3; }'
+assert 6 'int main() { return ({ 1; }) + ({ 2; }) + ({ 3; }); }'
+assert 3 'int main() { return ({ int x=3; x; }); }'
+
 echo OK

type.c

@@ -89,5 +89,17 @@ void add_type(Node *node) {
       error_tok(node->tok, "invalid pointer dereference");
     node->ty = node->lhs->ty->base;
     return;
+  case ND_STMT_EXPR:
+    if (node->body) {
+      Node *stmt = node->body;
+      while (stmt->next)
+        stmt = stmt->next;
+      if (stmt->kind == ND_EXPR_STMT) {
+        node->ty = stmt->lhs->ty;
+        return;
+      }
+    }
+    error_tok(node->tok, "statement expression returning void is not supported");
+    return;
   }
 }