[armedbear-devel] Fwd: [Bug] Identically named gensymed coalesced into same object in fasl file

[Erik Huelsmann]

Sent the response below to Tobias privately by accident.

---------- Forwarded message ----------
From: Erik Huelsmann <ehuels at gmail.com>
Date: Mon, Jan 11, 2010 at 11:40 PM
Subject: Re: [armedbear-devel] [Bug] Identically named gensymed
coalesced into same object in fasl file
To: "Tobias C. Rittweiler" <tcr at freebits.de>


Hi Tobias,

Thanks for your report. This issue has long been in ABCL and it's not fixed yet.

I have known about this problem for a while, but was kind of hoping it
wouldn't show up in practice. I guess it did.

This is what happens: when reading, the fasl loader creates an
anonymous package in which it interns uninterned symbols. It "has" to
do that, because ABCL doesn't store references to the actual unique
objects everywhere; instead it stores the textual representation in
some places. These symbols get coalesced by use of the anonymous
package.

So, generally the system of coalescing is something inherent to this
idea. The idea is wrong enough for you to run into and get bitten by
it.

We'll have to come up with a better idea. Can you add the mail message
below to our issue tracker?


BTW: Did you happen to find this issue by accident? How did you find it?


Bye,


Erik.


On Mon, Jan 11, 2010 at 10:35 PM, Tobias C. Rittweiler <tcr at freebits.de> wrote:
>
> I'm currently running an old checkout ("0.17.0-dev"), so the issue may
> have been fixed meanwhile.
>
>  ### /tmp/foo.lisp
>  (eval-when (:load-toplevel :execute)
>    (let ((a '#:foo)
>          (b '#:foo))
>      (setf (fdefinition 'foo) (lambda () (values a b)))))
>
>  (eval-when (:load-toplevel :execute)
>    (format t "~&(multiple-value-call #'eq (foo)) => ~S~%"
>            (multiple-value-call #'eq (foo))))
>  ###
>
>  CL-USER> (compile-file "/tmp/foo.lisp")
>  ; Compiling /tmp/foo.lisp ...
>  ; (LET (# #) ...)
>  ; (FORMAT T ...)
>
>  ; Compilation unit finished
>  ;   The following functions were used but not defined:
>  ;     FOO
>
>  ; Wrote /tmp/foo.abcl (0.034 seconds)
>  #P"/tmp/foo.abcl"
>  NIL
>  NIL
>
> That warning is bogus. There some work in this area so this bit may be
> fixed already.
>
> The actual issue of my mail is this one, though:
>
>  CL-USER> (load *)
>  (multiple-value-call #'eq (foo)) => T
>  T
>
> As you can see, ABCL coalesces the two identical named gensyms into the
> same object in the fasl file.
>
> While 3.2.4.2.2 does specify that
>
>  Two apparently uninterned symbols S and C are similar if their names
>  are similar.
>
> ABCL's behavior is still wrong because 3.2.4.1 says
>
>  If two literal objects appearing in the source code for a single file
>  processed with the file compiler are the [sic] identical, the
>  corresponding objects in the compiled code must also be the
>  identical. __With the exception of symbols and packages__, any two
>  literal objects in code being processed by the file compiler may be
>  coalesced if and only if they are similar; __if they are either both
>  symbols or both packages, they may only be coalesced if and only if
>  they are identical__.
>
>  -T.
>
>
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