How to get the default values for parameters?

[Hanke Zhang via Gcc]

Hi,
I'm trying to get the default values for parameters of some functions
in my GIMPLE-PASS. The example code is here:

enum {
  edefault1 = 1,
  edefault2 = 2,
  edefault3 = 3
}

void func(int p0, int p1 = edefault1, int p2 = edefault2, int p3 = edefault3) {
  // do other things
}

I want to get the value of `p1` here. But I didn't find a way to get it.

And I noticed that the marco `DECL_INITIAL` cannot be applied on
PARM_DECL. And the comments say that the default values for parameters
are encoded in the type of the function. But I can't find it.

If you know how to get the value, I'll be so grateful! :)

Thanks
Hanke Zhang