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bug#51982: Erroneous handling of local variables in byte-compiled nested
From: |
Mattias Engdegård |
Subject: |
bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas |
Date: |
Sun, 21 Nov 2021 10:59:15 +0100 |
21 nov. 2021 kl. 08.59 skrev Michael Heerdegen <michael_heerdegen@web.de>:
> If arbitrary fixes would be allowed at any point of the development cycle
> the result would be worse than what we have now. It's also a quite
> common policy.
Yes, but it does cause some inconvenience in a project with Emacs's geological
release pace. It is difficult to defend an inability to rapidly release an
updated version when a serious bug has been found.
Now, regarding the actual bug. Consider the function
1 (defun f (x)
2 (lambda ()
3 (let ((g (lambda () x)))
4 (let ((x 'a))
5 (list x (funcall g))))))
First of all, the variable x is free in the function starting in line 2, so
that function is converted to a closure capturing that variable explicitly.
Next, the function bound to g will be lambda-lifted; ie, converted to (lambda
(x) x) which means that the call to g in line 5 must be amended to include the
value of x. However, we can't just change (funcall g) to (funcall g x) because
x is shadowed by the binding in 4, so a new variable is introduced for this
purpose.
The result after cconv is essentially (without the fix):
1 (defun f (x)
2 (internal-make-closure nil (x) nil
3 (let ((g (lambda (x) x)))
4 (let ((x 'a)
5 (closed-x x))
6 (list x (funcall g closed-x))))))
But x is not the right expression for closed-x, because it is a captured
variable. The patch fixes this:
1 (defun f (x)
2 (internal-make-closure nil (x) nil
3 (let ((g (lambda (x) x)))
4 (let ((x 'a)
5 (closed-x (internal-get-closed-var 0)))
6 (list x (funcall g closed-x))))))
As I mentioned previously, it would be probably be better to elide closed-x
entirely and produce
1 (defun f (x)
2 (internal-make-closure nil (x) nil
3 (let ((g (lambda (x) x)))
4 (let ((x 'a))
5 (list x (funcall g (internal-get-closed-var 0)))))))
In other words, the bug occurs when a variable is captured, lambda-lifted, and
shadowed.
I need to take another good look at the code to make sure the change is correct
(more eyes on it would be appreciated).
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Paul Pogonyshev, 2021/11/19
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Michael Heerdegen, 2021/11/19
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Mattias Engdegård, 2021/11/20
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Paul Pogonyshev, 2021/11/20
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Michael Heerdegen, 2021/11/21
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas,
Mattias Engdegård <=
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Michael Heerdegen, 2021/11/22
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Mattias Engdegård, 2021/11/22
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Mattias Engdegård, 2021/11/22
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Stefan Monnier, 2021/11/30
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Mattias Engdegård, 2021/11/30
- bug#51982: Erroneous handling of local variables in byte-compiled nested lambdas, Stefan Monnier, 2021/11/30