[GNAT] This restriction prevents the compiler from building ’trampolines’.
This is a structure that is built on the stack and contains dynamic
code to be executed at run time. On some targets, a trampoline is
built for the following features: Access,
Unrestricted_Access, or Address of a nested subprogram;
nested task bodies; primitive operations of nested tagged types.
Trampolines do not work on machines that prevent execution of stack
data. For example, on windows systems, enabling DEP (data execution
protection) will cause trampolines to raise an exception.
Trampolines are also quite slow at run time.
On many targets, trampolines have been largely eliminated. Look at the
version of system.ads for your target — if it has
Always_Compatible_Rep equal to False, then trampolines are largely
eliminated. In particular, a trampoline is built for the following
features: Address of a nested subprogram;
Access or Unrestricted_Access of a nested subprogram,
but only if pragma Favor_Top_Level applies, or the access type has a
foreign-language convention; primitive operations of nested tagged
types.