Exceptions and out procedure arguments (using GNAT GPL)

[Fionn Mac Cumhaill <invisible@hiding.from.spam>]

Consider a procedure that starts like this:

procedure My_Procedure (
  O: out integer
)
is
begin

  -- various statements follow

  O := 999;

  -- more statements follow

  raise My_Exception;

I'm using GNAT GPL.

My question is:

Is the routine which calls My_Procedure guaranteed to get a value if
it does something like this?

X := 0;
My_Procedure(X);

and has an exception handler

exception
  when My_Exception =>
    null;

Will X get a value of 999? I.e., is an Ada compiler free to optimize
the code such that the assignment to the out variable O is moved to
some place after the point where the exception is raised?