How to create a plugin to instrument function calls

[Paul Sery]

Hi,

I want to create a plugin that inserts custom code before function calls. The
plugin would mimic the way fprofile-arcs instruments function calls, but not
other structures. For instance, I'd like to modify the following code,

..
void main(...) {
  f1();
  f2();
}

as follows,

..
void main(...) {
  _mystuff('f1');
  f1();
  _mystuff('f2');
  f2();
}

I think/guess the best place for this to occur is in the assembler, like
finstrument-functions? 

I've got the basic plugin mechanics working under gcc 4.5 - i.e.,
dumb_plugin.c from testsuite - and would appreciate any suggestions about how
to proceed from here.

Regards,
Paul