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[perl #123722] Dumper screws up aliasing of array elements

From:
Zefram
Date:
February 3, 2015 11:41
Subject:
[perl #123722] Dumper screws up aliasing of array elements
Message ID:
rt-4.0.18-16637-1422963680-375.123722-75-0@perl.org
# New Ticket Created by  Zefram 
# Please include the string:  [perl #123722]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=123722 >



This is a bug report for perl from zefram@fysh.org,
generated with the help of perlbug 1.39 running under perl 5.18.2.


-----------------------------------------------------------------
[Please describe your issue here]

Setting the Purity option is supposed to make Dumper emit correct
evalable code.  For example, it makes all the difference in this case:

$ perl -MData::Dumper=Dumper -we '@a=(3,0); $a[1] = \$a[0]; print Dumper \@a'
$VAR1 = [
          3,
          \$VAR1->[0]
        ];
$ perl -MData::Dumper=Dumper -we '$Data::Dumper::Purity=1; @a=(3,0); $a[1] = \$a[0]; print Dumper \@a'               
$VAR1 = [
          3,
          do{my $o}
        ];
$VAR1->[1] = \$VAR1->[0];

But if I put the same array elements the other way round, Dumper gets
it wrong even with the Purity option:

$ perl -MData::Dumper=Dumper -we '$Data::Dumper::Purity=1; @a=(0,3); $a[0] = \$a[1]; print Dumper \@a'               
$VAR1 = [
          \3,
          ${$VAR1->[0]}
        ];

The mention of $VAR1->[0] comes too early, before $VAR1 has been set to
an array ref, and the expression for $VAR1->[1] doesn't account for the
copying performed by the [...] array constructor.  Correct output would
have to be much like the correct output for the first array:

$VAR1 = [
          do{my $o},
          3
        ];
$VAR1->[0] = \$VAR1->[1];

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=library
    severity=low
    module=Data::Dumper
---
Site configuration information for perl 5.18.2:

Configured by root at Wed Mar 26 14:32:51 UTC 2014.

Summary of my perl5 (revision 5 version 18 subversion 2) configuration:
   
  Platform:
    osname=linux, osvers=3.2.0-4-amd64, archname=x86_64-linux-ld
    uname='linux ukmcwzefram.photobox.priv 3.2.0-4-amd64 #1 smp debian 3.2.54-2 x86_64 gnulinux '
    config_args='-des -Duseshrplib -Duse64bitint -Duselongdouble -Uusethreads -Uusemultiplicity -Dprefix=/opt/perl-5.18.2 -Dsiteprefix=/opt/perl-5.18.2 -Dvendorprefix=/opt/perl-5.18.2/vendor -Doptimize=-ggdb -O3 -fbranch-target-load-optimize -fgcse-las -fgcse-sm -fipa-pta -floop-block -floop-interchange -floop-strip-mine -fmodulo-sched -fomit-frame-pointer -freorder-blocks-and-partition -fsched-spec-load -fsched-spec-load-dangerous -ftree-loop-distribution -Dcccdlflags=-fPIC -O3 -pipe'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=define, use64bitall=define, uselongdouble=define
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
    optimize='-ggdb -O3 -fbranch-target-load-optimize -fgcse-las -fgcse-sm -fipa-pta -floop-block -floop-interchange -floop-strip-mine -fmodulo-sched -fomit-frame-pointer -freorder-blocks-and-partition -fsched-spec-load -fsched-spec-load-dangerous -ftree-loop-distribution',
    cppflags='-fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.7.2', gccosandvers=''
    intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=8, nvtype='long double', nvsize=16, Off_t='off_t', lseeksize=8
    alignbytes=16, prototype=define
  Linker and Libraries:
    ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib /usr/lib
    libs=-lnsl -ldb -ldl -lm -lcrypt -lutil -lc
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lc
    libc=, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.17'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E -Wl,-rpath,/opt/perl-5.18.2/lib/5.18.2/x86_64-linux-ld/CORE'
    cccdlflags='-fPIC -O3 -pipe', lddlflags='-shared -ggdb -O3 -fbranch-target-load-optimize -fgcse-las -fgcse-sm -fipa-pta -floop-block -floop-interchange -floop-strip-mine -fmodulo-sched -fomit-frame-pointer -freorder-blocks-and-partition -fsched-spec-load -fsched-spec-load-dangerous -ftree-loop-distribution -L/usr/local/lib -fstack-protector'

Locally applied patches:
    

---
@INC for perl 5.18.2:
    /opt/perl-5.18.2/lib/site_perl/5.18.2/x86_64-linux-ld
    /opt/perl-5.18.2/lib/site_perl/5.18.2
    /opt/perl-5.18.2/vendor/lib/vendor_perl/5.18.2/x86_64-linux-ld
    /opt/perl-5.18.2/vendor/lib/vendor_perl/5.18.2
    /opt/perl-5.18.2/lib/5.18.2/x86_64-linux-ld
    /opt/perl-5.18.2/lib/5.18.2
    .

---
Environment for perl 5.18.2:
    HOME=/home/zefram
    LANG (unset)
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/zefram/pub/x86_64-unknown-linux-gnu/bin:/home/zefram/pub/common/bin:/usr/bin:/bin:/usr/local/bin:/usr/games:/opt/babelhttpd-2.2.26/bin:/opt/babelhttpd-2.4.7/bin:/opt/geoip/bin:/opt/httpd/bin:/opt/perl/bin
    PERL_BADLANG (unset)
    SHELL=/usr/bin/zsh




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