GNU bug report logs - #19417
let, assq-set optimization gone wrong?

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Matt Wette <mwette <at> alumni.caltech.edu>
To: bug-guile <at> gnu.org
Cc: mwette <at> alumni.caltech.edu, Panicz Maciej Godek <godek.maciek <at> gmail.com>
Subject: let, assq-set optimization gone wrong?
Date: Fri, 19 Dec 2014 21:51:29 -0800

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This was discovered with help from godek.maciek <at> gmail.com.
It seems the variable "l" in the second let below is bound to the value from the first.
I ran into this bug when using syntax-rules with a let/a-list in the expansion.

mwette$ cat bug.scm
(let ((l '((a . 1) (b . 2)))) (assq-set! l 'b 99))
(let ((l '((a . 1) (b . 2)))) l)
mwette$ guile20
GNU Guile 2.0.11
Copyright (C) 1995-2014 Free Software Foundation, Inc.

Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
This program is free software, and you are welcome to redistribute it
under certain conditions; type `,show c' for details.

Enter `,help' for help.
scheme@(guile-user)> (load "bug.scm")
$1 = ((a . 1) (b . 99))

scheme@(guile-user)> %guile-build-info
$2 = ((buildstamp . "2014-10-07 02:36:10") (CFLAGS . " -I/opt/local/include -D_THREAD_SAFE ") (LIBS . "-L/opt/local/lib  -lm ") (libguileinterface . "29:2:7") (guileversion . "2.0.11") (extensiondir . "/opt/local/lib/guile/2.0/extensions") (pkgincludedir . "/opt/local/include/guile") (pkglibdir . "/opt/local/lib/guile") (pkgdatadir . "/opt/local/share/guile") (includedir . "/opt/local/include") (mandir . "/opt/local/share/man") (infodir . "/opt/local/share/info") (ccachedir . "/opt/local/lib/guile/2.0/ccache") (libdir . "/opt/local/lib") (localstatedir . "/opt/local/var") (sharedstatedir . "/opt/local/com") (sysconfdir . "/opt/local/etc") (datadir . "/opt/local/share") (libexecdir . "/opt/local/libexec") (sbindir . "/opt/local/sbin") (bindir . "/opt/local/bin") (exec_prefix . "/opt/local") (prefix . "/opt/local") (top_srcdir . "/opt/local/var/macports/build/_Users_mwette_proj_macports_ports_lang_guile20/guile20/work/guile-2.0.11") (srcdir . "/opt/local/var/macports/build/_Users_mwette_proj_macports_ports_lang_guile20/guile20/work/guile-2.0.11/libguile"))

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Message #8 received at 19417 <at> debbugs.gnu.org (full text, mbox):

From: Matt Wette <mwette <at> alumni.caltech.edu>
To: 19417 <at> debbugs.gnu.org
Subject: oops - may not be a bug
Date: Sat, 20 Dec 2014 06:44:50 -0800

I have been told this has to do with '((a . 1) (b . 1)) being a literal.   I'm guessing guile caches this and it get then mutated.

Message #11 received at 19417 <at> debbugs.gnu.org (full text, mbox):

From: Mark H Weaver <mhw <at> netris.org>
To: Matt Wette <mwette <at> alumni.caltech.edu>
Cc: 19417 <at> debbugs.gnu.org, request <at> debbugs.gnu.org
Subject: Re: bug#19417: oops - may not be a bug
Date: Mon, 22 Dec 2014 02:02:44 -0500

tags 19417 + notabug
close 19417
thanks

Matt Wette <mwette <at> alumni.caltech.edu> writes:
> I have been told this has to do with '((a . 1) (b . 1)) being a
> literal.  I'm guessing guile caches this and it get then mutated.

Right.  As explained on guile-user, mutating literal data is not
permitted and leads to unspecified behavior.  Our compiler aggressively
shares data between literals in the same compilation unit.

     Mark

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