Static Chain Argument in call_expr

[Duncan Sands]

> in tree.def, in DEFTREECODE for call_expr, it says operand 2 is the
> static chain argument, or NULL. Can someone tell me or reference me to
> what static chain argument is?

It's for nested functions, eg

int parent (int n)
{
  int child (int m) { return m * n; }

  return child (2);
}

Notice how child using a variable of parent, namely
the parameter n.  This gets lowered to something like:

struct frame { int n; };

int child (struct frame *static_chain, int m) { return m * static_chain->n; }

int parent (int n)
{
  struct frame FRAME;
  FRAME.n = n;
  return child (&FRAME, 2);
}

Ciao,

Duncan.