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Re: Add nested-function support in a language the based on a stack-machine

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On 2018-03-06, John Levine <johnl@taugh.com> wrote:
>>>>>This is because displays were found to be more costly for Algol-like
>>>>>languages ...
>>>
>>>>> IIRC the additional cost is in updating the display on calls
>>>>>and returns.
>
> I get the impression that displays describe two different things.
>
> One is a static array that has one entry for each level of lexical
> scoping.  In a routine declared N levels (zero based) down, its prolog
> saves the Nth entry in the display and replaces it with a pointer to
> the current stack frame, and the epilog restores it.  It can assume
> that higher level routines have correctly set entries 0:N-1.  That
> seems pretty efficient.  Assuming the saved pointer is at a known
> location in each stack frame you can do a longjmp and unwind stacks
> without too much pain.
>
> The other is the same array, but with a copy of the current display in
> each routine's stack frame.  This is slower at call time but probably
> faster to use on machines like S/360 without direct addressing
> since the display on the stack is addressable from the frame pointer,
> but static data needs to load a pointer from a constant pool.  A
> longjmp doesn't need anything special since stacked displays go away
> when the stack frames go away.

This is precisely what I happen to be working on at the moment: a
virtual machine for a Lisp dialect in which I plan to have displays
copied into the local stack frame.  I expect it to be a performance
advantage that there is nothing special to do when the stack frame goes
away.

If we had to take some unconditional action, we would have to set up
a catch frame. That means doing a setjmp-like operation that saves
a whole vector of stuff similar to a jmp_buf. Not just machine
registers, but the signal mask (or a virtualized representation of it,
for efficiency), and other stuff. This vector is larger than a display
display containing a significant number of environment levels!

If the scope binds dynamically scoped variables, those have to be
undone.  However, that requires no special action.  The dynamic
environment is a chain rooted at a global pointer called dyn_env.  Long
ago, I made the simplifying design decision to include this dyn_env
pointer in the jmp_buf-like context that is saved at every catch frame,
and so there is no need for a scope to set up a handler for the sake of
restoring dynamically scoped bindings. The normal return case has to
restore dyn_env, which is simple.

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Add nested-function support in a language the based on a stack-machine dror.openu@gmail.com - 2018-02-12 11:25 -0800
  Re: Add nested-function support in a language the based on a stack-machine dror.openu@gmail.com - 2018-02-12 14:16 -0800
    Re: Add nested-function support in a language the based on a stack-machine Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2018-02-13 12:10 +0100
  Re: Add nested-function support in a language the based on a stack-machine Louis Krupp <lkrupp@nospam.pssw.com.invalid> - 2018-02-13 00:42 -0700
    Re: Add nested-function support in a language the based on a stack-machine Kaz Kylheku <217-679-0842@kylheku.com> - 2018-02-14 00:59 +0000
      Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-02-13 22:04 -0500
        Re: Add nested-function support in a language the based on a stack-machine Kaz Kylheku <217-679-0842@kylheku.com> - 2018-02-14 18:06 +0000
          Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-02-15 11:41 -0500
            Re: Add nested-function support in a language the based on a stack-machine Kaz Kylheku <217-679-0842@kylheku.com> - 2018-02-17 16:13 +0000
              Re: Add nested-function support in a language the based on a stack-machine anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2018-02-17 16:39 +0000
                Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-03-01 13:48 -0500
                Re: Add nested-function support in a language the based on a stack-machine Kaz Kylheku <217-679-0842@kylheku.com> - 2018-03-01 19:26 +0000
                Re: Add nested-function support in a language the based on a stack-machine Kaz Kylheku <217-679-0842@kylheku.com> - 2018-03-02 00:38 +0000
                Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-03-02 02:48 -0500
                Re: Add nested-function support in a language the based on a stack-machine rpw3@rpw3.org (Rob Warnock) - 2018-03-03 16:00 +0000
                Re: Add nested-function support in a language the based on a stack-machine anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2018-03-02 09:30 +0000
                Re: Add nested-function support in a language the based on a stack-machine anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2018-03-05 15:01 +0000
                Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-03-05 15:51 -0500
                Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-03-05 14:39 -0500
                Re: Add nested-function support in a language the based on a stack-machine John Levine <johnl@taugh.com> - 2018-03-06 04:31 +0000
                Re: Add nested-function support in a language the based on a stack-machine Kaz Kylheku <217-679-0842@kylheku.com> - 2018-03-06 18:17 +0000
                Re: Add nested-function support in a language the based on a stack-machine Kaz Kylheku <157-073-9834@kylheku.com> - 2018-03-11 03:19 +0000
                Re: Add nested-function support in a language the based on a stack-machine Tomasz Kowaltowski <tk@ic.unicamp.br> - 2018-03-06 12:10 -0300
                Re: Add nested-function support in a language the based on a stack-machine bartc <bc@freeuk.com> - 2018-03-06 19:02 +0000
                Re: Add nested-function support in a language the based on a stack-machine antispam@math.uni.wroc.pl - 2018-04-29 16:29 +0000
              Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-03-01 13:49 -0500
    Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-02-13 22:37 -0500
  Re: Add nested-function support in a language the based on a stack-machine George Neuner <gneuner2@comcast.net> - 2018-02-13 23:27 -0500
  Re: Add nested-function support in a language the based on a stack-machine Shoefoot <shoefoot@gmail.com> - 2018-02-14 12:27 -0800

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