Re: A different stack discipline

[Mikael Djurfeldt <mikael@djurfeldt.com>]

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On Sat, Nov 3, 2018 at 4:30 PM Hugo Hörnquist <hugo@lysator.liu.se> wrote:

> The section, as far as I can see, just describes a machine
> which pushes continuation instead of the PC counter to the
> stack.
>
> Also, while in theory quite nice it has the problem that
> Guile is really slow in restoring continuations, due to the
> fact that we have complete C interoperability.
>

There's some misunderstanding here. The SICP register machine model is not
very different from common register machine models. There's just a
difference in how to handle subroutine calls. A short example:

Let's first write out all operations involved in a call in a conventional
register machine:

        [...]
        ; The following three micro operations consitute "call foo ()"
        (sp) <- pc + offset(L1) ; NOTE the external memory access
        sp <- sp - 1
        pc <- pc + offset(foo)
L1:    [...]

foo:   [...]
        ; the following two micro operations constitute "ret"
        sp <- sp + 1
        pc <- (sp) ; NOTE the external memory access

Now look at the call in the SICP register machine:

        [...]
        continue <- pc + offset(L1)
        pc <- pc + offset(foo)
L1:   [...]

foo:  [...]
        pc <- continue

It is fewer operations and every operation is immediate with no memory
access. I *have* cheated since I omit a need to push the continue register
onto the stack, but while this is needed at *every* call for the
conventional machine, this is only required once at the beginning of a
function in the SICP machine *unless* the function has a tail call, in
which case we don't need to push anything. So, while one can say that we
only "push around the pushes", we make gains for every tal call.

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