Re: [9fans] useful language extension, or no?

[forsyth@vitanuova.com]

it only uses the trampoline code when you call g indirectly through
a function pointer.  if you call it directly it can provide the static link
itself without that fuss.   when you create a function pointer to g,
as in the call to printf, it does generate code onto the stack that
loads the static link register then enters g.
	leal -16(%ebp),%eax
	movl $g.9-10,%edx
	movb $185,(%eax)
	movl %ebp,1(%eax)
	movb $233,5(%eax)
	subl %eax,%edx
	movl %edx,6(%eax)
that's the generation on the fly of the trampoline code.
note the movl %ebp,1(%eax), which stores the current frame pointer
as a literal in the generated code.
the `function pointer' that is later passed to printf is the address of
-16(%ebp) [offsets might be different in your case], which when
called will load the literal, and enter g with a pc-relative reference
(i think that's about right).

you'd need to flush the cache(s) on some architectures, which is
tricky when it's a privileged operation.

the problem it is trying to solve is to get the right value for the static link register,
without passing the required value along with the function pointer as a pair.
providing the right value for the static link register is
easiest to do if it doesn't exist.  there are several
approaches to implementing displays, and one of them allows nested
function pointers as a single word but without this crud.

all these implementations of nested functions in C go sadly wrong if
you call a nested function indirectly when it has gone out of scope.