Help with creating a plugin that modifies a program

[Wissen <aka-wissen@xxxxxxxxx>]

Is it possible for GCC plugin to modify program (at AST level) so that program behavior changes?
For instance, how to write plugin to implicitly declare global variable "fake_var" in the following program?:

// fake.c
int main () {
  return fake_var;
}

When compiled with "gcc -fplugin=myplugin.so -o fake fake.c" there should be no syntax error messages.
GCC version used: 4.5.0

I tried the following plugin code from testsuite, but it does nothing:
tree fake_var = build_decl (UNKNOWN_LOCATION, VAR_DECL, get_identifier ("fake_var"), integer_type_node);
TREE_PUBLIC (fake_var) = 1;
DECL_ARTIFICIAL (fake_var) = 1;

(Event: PLUGIN_PRE_GENERICIZE, don't want to use gimple)