Re: please help concerning specpdl

[Alin Soare <as1789@gmail.com>]

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> > count = SPECPDL_INDEX ();
> > record_unwind_protect
> > val = Fprogn (args);
> > return unbind_to (count, val);
>
> > Can you explain me the logic of specpdl please ?
>
> The specpdl contains information needed to remove variable bindings and
> provide other services that are guaranteed even in the case of abnormal
> exit.  Abnormal exits operate by calling longjmp(3), so there's no
> guaranteed chance to perform cleanup in the normal flow of execution.  But
> the function that calls longjmp(3) (`unwind_to_catch') first peruses the
> specpdl and handles its elements appropriately, so the guaranteed
> operations happen.  The actual unwinding is done by `unbind_to', so if the
> body exits normally, we just call that to perform the unwinding operations
> without longjmp(3).
>

I think I understand the analogy with longjmp. It is a stack of Lisp
Objects, and this stack is used by C code to protect against errors in lisp
interpreter.

The variables defined by let are memorized in this stack using SAFE_ALLOCA.

Why let-binding-variables are memorized exactly here and not elsewhere ?

On the other hand, the macro SAFE_ALLOCA is defined as

do{
}while (0)

why the do-while is good in this case, and not simply brackets?



> So: the count identifies how far to unwind in case of normal exit (in case
> of abnormal exit, we'll unwind farther anyway, so we don't need `count').
> The `record_unwind_protect' registers something to do at unwind-time, and
> then `unbind_to' performs it unless `unwind_to_catch' does.  `val' is
> passed to `unbind_to' for GC reasons, I believe.
>

I see that inside unbind_to, the symbols are unbounded 1 by one. Why the
specpdl_ptr is not decremented directly with count ?

while (specpdl_ptr != specpdl + count)

Probably because unbind_to is called from lisp code by (throw 'symbol
value), and specpdl_ptr must decrement 1 by 1 until the 'symbol is dound on
the stack ?

Apart from (throw ... ), is which other situation unbind_to is called ?


I see that GCPROx macros are used to protect the variables of type Lisp
Object on the stack of C code (that the compiler creates), not to protect
the lisp objects in specpdl. The GCPRO protection is against the algorthm of
conservative stack. Am I right ?


Sorry, I think I understand something, but I am far from enough.


Alin.

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