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

George Neuner <gneuner2@comcast.net>
Tue, 13 Feb 2018 22:04:29 -0500

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From: George Neuner <gneuner2@comcast.net>
Newsgroups: comp.compilers
Date: Tue, 13 Feb 2018 22:04:29 -0500
Organization: A noiseless patient Spider
References: 18-02-009 18-02-012 18-02-016
Keywords: code

On Wed, 14 Feb 2018 00:59:34 +0000 (UTC), Kaz Kylheku <217-679-0842@kylheku.com> wrote:
>On 2018-02-13, Louis Krupp <lkrupp@nospam.pssw.com.invalid> wrote:
>> On Mon, 12 Feb 2018 11:25:36 -0800 (PST), dror.openu@gmail.com wrote:
>>
>>>Suppose I have a simple C-like programming language: ...
>>>Like you can see, it supports nested functions.
>>
>> gcc supports nested functions as an extension to C. Compiling this
>> program with -O0 -fdump-tree-all and looking at the generated files
>> might give you an idea of one way to do it: ...


>> Apparently, gcc chains stack frames, and accessing uplevel variables,
>> as when p3 uses k1 and k2, seems like it would take O(n) time, where n
>> is the nesting level.
>>
>> The alternative, a display vector, seems like it would be easier to
>
>The problem is that these functions can recurse, including mutually.
>So that is to say, you can end up with a situation in which you have
>a single activation of main with a single k1 variable, but multiple
>activations of p2, with multiple k2's all being live at the
>same time.
>
>To complicate things, the multiple active p2 functions can be lifted
>into function pointers, all of which are simultaneously valid.
>
>Each of those p2 pointers could be called back by some external function.
>Those calls have to establish an environment which resolves to the
>correct k2, and at the same time to the one and only k1.
>
>That doesn't rule out display vectors, but it seems like each function
>activation has to have its own complete display vector.


As long as each function creates a new stack frame when called, then
recursion is not relevant and a single display is sufficient to handle
the nesting.


The display tracks lexical parentage [who defined who], not call
chain parentage. Each display entry points to the stack frame of the
last function invoked that was defined at the corresponding lexical
level.


The preamble of a function at, e.g., level 3, preserves display[3] in
its own stack frame, and sets display[3] to point to its own frame.
The postscript of the function restores the original display[3] value
when the function ends.
[Obviously, the compiler must keep track of lexical levels in order
that each function update the proper display entry.]


When a function at level 4 needs something at level 2, it just looks
to display[2] to find the proper stack frame. Regardless of
intervening recursions, the lexical environment is maintained:
display[2] always will point to the latest invocation of the level 2
function which contains the current (level 4) function.




>For instance the p2 level functions need a two-level vector. Under
>the same main invocation, they share the vec[0] entry pointing to
>the main frame; but each one has a different vec[1] referencing its
>own frame.
>
>When a p3 executes it has its own 3 element vec[]. The vec[0] and
>vec[1] are copied from the parent p2. If a pointer to a p3 is captured,
>then its vec[1] correctly references the particular dynamic p2 instance
>in which it is scoped; vec[2] references its own environment.
>
>Under this scheme, we basically we need o(n^2) storage for the vectors,
>for n nesting levels in the absence of recursion.


No, you're missing something. I just haven't figured out what.


>The compiler can determine which functions aren't subject to indirection
>and avoid allocating the trampoline space for that.


With the classic implementation, every non-leaf function has to do its
part to maintain the lexical environment. But the allocation overhead
is just one saved pointer per stack frame.




>[As I recall, you do need a display with a pointer for each lexical scope,
>but so long as you don't expect closures to work, you can do it in
>constant time with a fixed location where each scope stores its
>current frame pointer. If you want closures, then you're into tree
>shaped stacks and garbage collected stack frames. -John]


You still do have closures of sorts ... they just are distributed
through the stack frames of the current function call chain.


A display cannot handle persistent closures, such as in Lisp, where a
nested function may be invoked after its parent terminates, outside of
the runtime environment that defined it.


George


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