ARC + goto into protected scope error

Clang Frontend
#1

Hi all,

I use the "check for success/goto failed" pattern everywhere, but
Clang refuses to compile such code with ARC enabled if the goto
bypasses a variable declaration (giving a "goto into protected scope"
error.)

Is this something that's likely to be fixed? Compiling the program
below triggers the error.

    // compile with: clang -framework Foundation -fobjc-arc <file>.m
    #import <Foundation/Foundation.h>
    int main() {
        goto cleanup;
        NSString *str;
        cleanup:
        return 0;
    }

#2

If you put the variable in its own scope, preventing accidental access, you won't get the error:

int main() {
  goto cleanup;
  {
     NSString *str;
  }
cleanup:
  return 0;
}

If you don't do this, you bypass the zero-initialization of 'str', and when ARC tries to clean it up, it'll end up trying to release some random bit of memory.

This should handle any valid case; if you want to use 'goto' but can't enclose the variables in their own scope, you'll have to move the declarations before the goto (as K&R intended *grin*).

Jordan

#3

Note that (IIRC) goto can /leave/ a protected scope, so you can also just put one set of braces around the /rest/ of your function, and only leave the cleanup code outside. You won't need to nest new braces after every goto.

#4

This restriction is not likely to be lifted; there are a number of challenges
with branches that enter scopes.

I assume your failure label is always at the end of the function; in that case,
you can wrap everything up to that point in curly braces, like so:

- (void) broadcastChanges {
  {
    if (self.error) goto failure;
    NSString *foo = self.name;
    [self broadcastName: foo];
    return;
  }
  failure:
  abort();
}

You might also consider moving to a structural programming pattern.

John.