Re: LLVM--Low Level Virtual Machine--and Ada

["Randy Brukardt" <randy@rrsoftware.com>]

"Duncan Sands" <baldrick@free.fr> wrote in message
news:mailman.8.1185305754.3834.comp.lang.ada@ada-france.org...
> Hi Randy,
>
> > > how do these thunks work?
> >
> > Generally, the compiler passes the address of the thunk to wherever it
is
> > needed, and it is called indirectly. The thunks themselves adjust the
> > parameters as needed and call the real routine.
>
> presumably pointers to nested functions are "fat pointers" containing
> both the identity of the nested function (equivalently, the identity of
> a "thunk" that knows about the nested function) and also some kind of
> pointer into the stack of the parent of the nested function.  Is that
> right?

Janus/Ada uses displays rather than static links (that was originally a
requirement of the University project that led to Janus/Ada, and we never
changed it), so in most cases, you don't need any special stack pointers.
(You only need them when you call to a place that is outside of the normal
nesting of subprograms.) There are some cases when you do need them, and in
those cases we need to provide a replacement display. That is usually saved
in a well-known place (i.e., for shared generics it is part of the generic
data block) and shared with a number of related thunks, although for
anonymous-access-to-subprogram it will be part of the pointer. Even in these
cases, we determine in the thunk how much of the display needs to be
replaced (it is often none) so as to keep the overhead to a minimum, and all
of this code is part of the thunk, not part of the call site.

                              Randy.