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Re: [Emacs-diffs] feature/byte-unwind-protect 916094a 2/2: Add new bytec

From: Stefan Monnier
Subject: Re: [Emacs-diffs] feature/byte-unwind-protect 916094a 2/2: Add new bytecodes for unwind-protect
Date: Tue, 23 Jan 2018 10:35:31 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

> +  (if (not byte-compile--use-old-handlers)
> +      (let ((except-tag (byte-compile-make-tag)))
> +    (pcase (cddr form)
> +      (`(:fun-body ,f)
> +       (byte-compile-form
> +        (if byte-compile--use-old-handlers `(list (list 'funcall ,f)) f)))
> +      (handlers
> +       (if byte-compile--use-old-handlers

Clearly the last two tests of byte-compile--use-old-handlers can be
dropped since they're inside a branch where we know
byte-compile--use-old-handlers is t.

BTW, when introducing byte-codes we've "traditionally" done it in two steps:
- in release N we add the code which handles the new byte-codes,
  and we add support for the new byte-code in the byte-compiler, but
  disabled by default.
- in release N+1 we enable the use of the new byte-codes in the
  byte-compiler by default.

This is done so as to reduce the pain when running a file compiled with
another Emacs version.

But we don't want to set byte-compile--use-old-handlers back to nil for
your new bytecodes, so better use another variable.

Also byte-compile--use-old-handlers can be retired, I think, because it
has played its role.

> +     CASE (Bpushunwindprotect): /* New in 27.1.  */
> +       {
> +         struct handler *c = push_handler (Qt, CATCHER_ALL);

Hmm... using this "catch+rethrow" method interferes with the
debug-on-error stacktrace:

    (defun sm-test-1 () (unwind-protect (sm-test-2) (message "hello")))
    (defun sm-test-2 () (message "test-1-in") (car 4))
    (setq debug-on-error t)

then call `sm-test-1` and you'll see that Emacs gives you a backtrace
that gives the impression the error was signaled in `sm-test-1`!

I also wonder if there can be places where we call something like
`unbind_to` which should run those new unwind-forms but won't find it
because it won't look at the handlers [ offhand, I can't think of any
place where that would happen, and I think it's probably OK, but
I can't quite convince myself that it definitely is.  ]

All in all, I'd really prefer if we kept the unwind forms in
the specpdl.  Was there a particular reason why you used the
handlers instead?


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