Question

Is there a real performance gain when I turn {$IMPORTEDDATA} off ?

The manual only says this: “The {$G-} directive disables creation of imported data references. Using {$G-} increases memory-access efficiency, but prevents a packaged unit where it occurs from referencing variables in other packages.”

---

Update:

Here is more info I could find:

“The Debugging section has the new option Use imported data references (mapped to $G), which
controls the creation of imported data references (increasing memory efficiency but preventing the
access of global variables defined in other runtime packages)”

Answer 1

Almost never

This directive only refers to accessing global unit variables from another unit.

If you use {$G+}

unit1;

interface

var
  Global1: integer;   //<--  this is a global var in unit1.
  Form1: TForm1;      //<--  also a global var, but really a pointer

Global1 will be accessed indirectly via a pointer (if and when accessed from outside unit1)
Form1 will also be accessed indirectly (i.e. change from a direct pointer to an indirect pointer).

if you use {$G-}, the access to integer global will be direct and thus slightly faster.

This will only make a difference if you use global public unit variables in another unit and in time critical code, i.e. almost never.

See this article: http://hallvards.blogspot.com/2006/09/hack13-access-globals-faster.html