I would like to propose and discuss two small features below:

1) Protection against C stack overflow caused by enormously huge
    `max-lisp-eval-depth'.  Note that the stack size limit is examined
    just once (because doing it each time when eval_sub or funcall is
    called introduces too much overhead), but some OSes are capable to
    change this limit when the program is running (prlimit on Linux,
    for example).

2) Capability to shrink specpdl stack if it becomes too large.  When
    `max-specpdl-size' is 83200000 and `max-lisp-eval-depth' is 640000,
    this extreme example with 10K let bindings:

    (defun f ()
      (let ((x0 0)
            (x1 1)
            ...
            (x9999 9999)) (f)))

    creates 73622255-slots specpdl stack before running out of C stack
    on my system, which results in 2.5G RSS.  And currently there is no
    way to reduce it back to reasonable size.

Dmitry