Make "main" to implicitly return 0

codegen.c

@@ -1284,6 +1284,13 @@ static void emit_text(Obj *prog) {
     gen_stmt(fn->body);
     assert(depth == 0);
 
+    // [https://www.sigbus.info/n1570#5.1.2.2.3p1] The C spec defines
+    // a special rule for the main function. Reaching the end of the
+    // main function is equivalent to returning 0, even though the
+    // behavior is undefined for the other functions.
+    if (strcmp(fn->name, "main") == 0)
+      println("  mov $0, %%rax");
+
     // Epilogue
     println(".L.return.%s:", fn->name);
     println("  mov %%rbp, %%rsp");

test/function.c

@@ -366,5 +366,4 @@ int main() {
   ASSERT(5, (***add2)(2,3));
 
   printf("OK\n");
-  return 0;
 }