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Re: Does recent great work on separating the bytecode stack make it easi
From: |
Mattias Engdegård |
Subject: |
Re: Does recent great work on separating the bytecode stack make it easier to show bytecode offsets in a traceback? |
Date: |
Mon, 2 May 2022 11:10:14 +0200 |
30 apr. 2022 kl. 18.25 skrev Rocky Bernstein <rocky@gnu.org>:
> I especially like and appreciate the comment in the C code in bytecode.c
> showing the Bytecode interpreter stack.
Many thanks for your kind words. Mustn't let an opportunity for making ASCII
art go to waste!
> Now that we have a more normal bytecode stack, we have "saved_pc" shouldn't
> it be possible to easily show the bytecode offset in
> lisp/emacs-lisp/backtrack.el ?
That information is indeed accessible now, at least in principle; the stack
format was designed with backtracing in mind. How to make effective use of it
is another matter. A bytecode offset to source location mapping is perhaps more
feasible now (with position-carrying symbols) but still a fair amount of work.
There are also questions about whether to carry around such a mapping all the
time (memory cost etc). I like the idea of rematerialising it when needed but
Elisp isn't easily conducive to that approach.
On the other hand, just exposing the bytecode offset in tracebacks could very
well be useful on its own for users who can read disassembled bytecode.
Regarding the stack trace information, there is some redundancy that could be
exploited: the backtrace recorded in the specpdl could probably be eliminated
for calls into bytecode (and with some gymnastics, for other calls as well).
Experiments have indicated that there are performance gains to be had here.
> Prior work on this can be found in
> https://lists.gnu.org/archive/html/emacs-devel/2020-07/msg00711.html
Thank you, this provides some interesting perspective. Some things have changed
since but the big problems remain more or less the same.
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