Why `@nospecialize(ex)` in macros?

General Usage Performance
1

Why do Julia experts use @nospecialize(ex) in functions used by macros?

E.g. here: Typed globals: macro to define multiple, without manual type annotations - #2 by simeonschaub

I’d say the only types the argument could be are Expr and LineNumberNode, so there’s not a whole lot to optimize by avoiding type specialization.
(Maybe ex could also be some ‘atom’/literal type I guess)

2

The short answer is because:

  • the type of ex isn’t known when @typed ex is called
  • it’s not very important to specialize rewrite_assignment on ex

And so for this kind of case @nospecialize can avoid runtime dispatch on rewrite_assignment and will slightly improve the macro expansion of @typed (although it may not be very important given that the computation happens at lowering time).

Well, rewrite_assignment is defined in a way that it can take arbitrary AST elements other than Expr and LineNumberNode e.g. Int, Symbol. I agree with that it may not be very important to optimize the computation that only happens at lowering time, but it’s certainly better to have @nospecialze there.

2 Likes
3

If you want to have a more understanding on what @nospecialize really does, the description of this PR may give some idea.

2 Likes