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Subject: |
Abolish max-specpdl-size |
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Date: |
Sun, 18 Sep 2022 13:57:22 +0200 |
This is a proposal to abolish max-specpdl-size, the user-controllable limit on
the internal specpdl stack.
It is safe to do so because unbounded specpdl growth is not possible without
proportionally growing the lisp-eval-depth. Therefore, max-lisp-eval-depth
suffices as a guard against runaway recursion and unreasonable memory usage as
it effectively bounds the specpdl stack as well.
The change would give us some performance benefits but the biggest gain is in
user convenience: currently, any code that needs deeper recursion than
permitted by default has to set two variables. What values to use is often a
matter of puzzlement and guessing since it is far from clear to anyone how they
relate.
Why abolish max-specpdl-size rather than max-lisp-eval-depth, if they are
connected? First and foremost, because the lisp-eval-depth is easier to explain
and understand; the specpdl is an internal implementation detail that most Lisp
users don't really have a firm grip on.
Moreover, although each Lisp call currently consumes at least one specpdl
entry, this won't necessarily remain true in the future: it should be possible
to eliminate that specpdl usage for gains in both speed and memory. In that
case, max-specpdl-size would no longer serve as an effective recursion limit.
Concretely, the change would amount to retaining max-specpdl-size as a plain
dynamic Lisp variable with no special meaning. We could obsolete it or keep it
around indefinitely. A patch will come -- it's all very straightforward.
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Subject: |
Re: bug#57911: Abolish max-specpdl-size |
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Date: |
Mon, 19 Sep 2022 19:36:51 +0200 |
19 sep. 2022 kl. 15.54 skrev Eli Zaretskii <eliz@gnu.org>:
>> And I take from Stefan's
>> response that he has no objections... Eli, any comments?
>
> No.
Thank you, pushed to master and closing.
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