Optimizer isn't optimizing read-only value decisions

[p5pRT]

Migrated from rt.perl.org#123370 (status was 'open')

Searchable as RT123370$

[p5pRT]

From @khwilliamson

This is a bug report for perl from khw@​khw.(none),
generated with the help of perlbug 1.40 running under perl 5.21.7.

---

I noticed that expressions that contain read-only variables such as $^V
or $], etc don't seem to get constant-folded, so for example

  if($] < 5.007) {
  FOO()
  } elsif ($] >= 5.007003) {
  BAR()
  } else {
  BAZ()
  }

should just have generated unconditional code that calls BAR(), with
everything else gone.

I was surprised to find this, because I thought the optimizer handled
such things, and this seems like a blatant oversight.

---

---

Flags​:
  category=core
  severity=low

---

Site configuration information for perl 5.21.7​:

Configured by khw at Fri Dec 5 08​:31​:42 MST 2014.

Summary of my perl5 (revision 5 version 21 subversion 7) configuration​:
  Commit id​: b173165
  Platform​:
  osname=linux, osvers=3.13.0-40-generic,
archname=x86_64-linux-thread-multi-ld
  uname='linux khw 3.13.0-40-generic #69-ubuntu smp thu nov 13
17​:53​:56 utc 2014 x86_64 x86_64 x86_64 gnulinux '
  config_args='-des -Uversiononly -Dprefix=/home/khw/blead -Dusedevel
-D'optimize=-ggdb3' -A'optimize=-ggdb3' -A'optimize=-O0'
-Accflags='-DPERL_BOOL_AS_CHAR' -Dman1dir=none -Dman3dir=none
-DDEBUGGING -Dcc=g++ -Dusemorebits -Dusethreads'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=define, usemultiplicity=define
  use64bitint=define, use64bitall=define, uselongdouble=define
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='g++', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DPERL_BOOL_AS_CHAR
-fwrapv -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector
-I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64
-D_FORTIFY_SOURCE=2',
  optimize=' -ggdb3 -O0',
  cppflags='-D_REENTRANT -D_GNU_SOURCE -DPERL_BOOL_AS_CHAR -fwrapv
-DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector
-I/usr/local/include'
  ccversion='', gccversion='4.8.2', gccosandvers=''
  intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678,
doublekind=3
  d_longlong=define, longlongsize=8, d_longdbl=define,
longdblsize=16, longdblkind=3
  ivtype='long', ivsize=8, nvtype='long double', nvsize=16,
Off_t='off_t', lseeksize=8
  alignbytes=16, prototype=define
  Linker and Libraries​:
  ld='g++', ldflags =' -fstack-protector -L/usr/local/lib'
  libpth=/usr/include/c++/4.8 /usr/include/x86_64-linux-gnu/c++/4.8
/usr/include/c++/4.8/backward /usr/local/lib
/usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed
/usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib
/usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
  libs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
  perllibs=-lnsl -ldl -lm -lcrypt -lutil -lpthread -lc
  libc=libc-2.19.so, so=so, useshrplib=false, libperl=libperl.a
  gnulibc_version='2.19'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
  cccdlflags='-fPIC', lddlflags='-shared -ggdb3 -ggdb3 -O0
-L/usr/local/lib -fstack-protector'

---

@​INC for perl 5.21.7​:
  /home/khw/perl/blead/lib
  /home/khw/blead/lib/perl5/site_perl/5.21.7/x86_64-linux-thread-multi-ld
  /home/khw/blead/lib/perl5/site_perl/5.21.7
  /home/khw/blead/lib/perl5/5.21.7/x86_64-linux-thread-multi-ld
  /home/khw/blead/lib/perl5/5.21.7
  /home/khw/blead/lib/perl5/site_perl/5.21.6
  /home/khw/blead/lib/perl5/site_perl/5.21.5
  /home/khw/blead/lib/perl5/site_perl/5.21.4
  /home/khw/blead/lib/perl5/site_perl/5.21.3
  /home/khw/blead/lib/perl5/site_perl/5.21.2
  /home/khw/blead/lib/perl5/site_perl/5.21.1
  /home/khw/blead/lib/perl5/site_perl/5.20.0
  /home/khw/blead/lib/perl5/site_perl/5.19.12
  /home/khw/blead/lib/perl5/site_perl/5.19.11
  /home/khw/blead/lib/perl5/site_perl/5.19.10
  /home/khw/blead/lib/perl5/site_perl
  .

---

Environment for perl 5.21.7​:
  HOME=/home/khw
  LANG=en_US.UTF-8
  LANGUAGE (unset)
  LD_LIBRARY_PATH (unset)
  LOGDIR (unset)

PATH=/home/khw/bin​:/home/khw/perl5/perlbrew/bin​:/home/khw/print/bin​:/bin​:/usr/local/sbin​:/usr/local/bin​:/usr/sbin​:/usr/bin​:/sbin​:/usr/games​:/usr/local/games​:/home/khw/iands/www​:/home/khw/cxoffice/bin
  PERL5OPT=-w
  PERL_BADLANG (unset)
  PERL_POD_PEDANTIC=1
  SHELL=/bin/ksh

[p5pRT]

From @cpansprout

On Fri Dec 05 12​:33​:28 2014, public@​khwilliamson.com wrote​:

This is a bug report for perl from khw@​khw.(none),
generated with the help of perlbug 1.40 running under perl 5.21.7.

-----------------------------------------------------------------
I noticed that expressions that contain read-only variables such as
$^V
or $], etc don't seem to get constant-folded, so for example

if($] < 5.007) {
FOO()
} elsif ($] >= 5.007003) {
BAR()
} else {
BAZ()
}

should just have generated unconditional code that calls BAR(), with
everything else gone.

I was surprised to find this, because I thought the optimizer handled
such things, and this seems like a blatant oversight.

$ perl -le 'local $]; $]=3; print $]'
3

That kind of thing can be useful for testing porpoises (and dolphins).

--

Father Chrysostomos

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

From @khwilliamson

On 12/05/2014 01​:41 PM, Father Chrysostomos via RT wrote​:

On Fri Dec 05 12​:33​:28 2014, public@​khwilliamson.com wrote​:

This is a bug report for perl from khw@​khw.(none),
generated with the help of perlbug 1.40 running under perl 5.21.7.

-----------------------------------------------------------------
I noticed that expressions that contain read-only variables such as
$^V
or $], etc don't seem to get constant-folded, so for example

if($] < 5.007) {
FOO()
} elsif ($] >= 5.007003) {
BAR()
} else {
BAZ()
}

should just have generated unconditional code that calls BAR(), with
everything else gone.

I was surprised to find this, because I thought the optimizer handled
such things, and this seems like a blatant oversight.

$ perl -le 'local $]; $]=3; print $]'
3

That kind of thing can be useful for testing porpoises (and dolphins).

But if you do that, your local $] isn't marked read-only, so when a
variable is read-only, it could be constant-folded at compile time

[p5pRT]

From @cpansprout

On Fri Dec 05 12​:57​:57 2014, public@​khwilliamson.com wrote​:

On 12/05/2014 01​:41 PM, Father Chrysostomos via RT wrote​:

On Fri Dec 05 12​:33​:28 2014, public@​khwilliamson.com wrote​:

This is a bug report for perl from khw@​khw.(none),
generated with the help of perlbug 1.40 running under perl 5.21.7.

-----------------------------------------------------------------
I noticed that expressions that contain read-only variables such as
$^V
or $], etc don't seem to get constant-folded, so for example

if($] < 5.007) {
FOO()
} elsif ($] >= 5.007003) {
BAR()
} else {
BAZ()
}

should just have generated unconditional code that calls BAR(), with
everything else gone.

I was surprised to find this, because I thought the optimizer handled
such things, and this seems like a blatant oversight.

$ perl -le 'local $]; $]=3; print $]'
3

That kind of thing can be useful for testing porpoises (and dolphins).

But if you do that, your local $] isn't marked read-only, so when a
variable is read-only, it could be constant-folded at compile time

But code already compiled will be bound to *] glob and see the localisation change.

I’m not saying we shouldn’t necessarily optimise here; I’m just afraid of the aftermath. After all, simpler, more ‘internal’ changes I make seem to break more CPAN modules than I would like.

--

Father Chrysostomos

[p5pRT]

From @demerphq

On 5 December 2014 at 21​:41, Father Chrysostomos via RT <
perlbug-followup@​perl.org> wrote​:

On Fri Dec 05 12​:33​:28 2014, public@​khwilliamson.com wrote​:

This is a bug report for perl from khw@​khw.(none),
generated with the help of perlbug 1.40 running under perl 5.21.7.

-----------------------------------------------------------------
I noticed that expressions that contain read-only variables such as
$^V
or $], etc don't seem to get constant-folded, so for example

if($] < 5.007) {
FOO()
} elsif ($] >= 5.007003) {
BAR()
} else {
BAZ()
}

should just have generated unconditional code that calls BAR(), with
everything else gone.

I was surprised to find this, because I thought the optimizer handled
such things, and this seems like a blatant oversight.

$ perl -le 'local $]; $]=3; print $]'
3

Wow. I am shocked that works.

Yves

--
perl -Mre=debug -e "/just|another|perl|hacker/"

[p5pRT]

From @bulk88

On Fri Dec 05 14​:10​:18 2014, sprout wrote​:

But code already compiled will be bound to *] glob and see the
localisation change.

I’m not saying we shouldn’t necessarily optimise here; I’m just afraid
of the aftermath. After all, simpler, more ‘internal’ changes I make
seem to break more CPAN modules than I would like.

What if someone bites the bullet and uploads "Perl6.pm" to CPAN that just changes $^V and $] to 6 and nothing else? :D

We would break that code then.

More realistically, "If you expect to change $^V and $] for testing please do so in a BEGIN block, runtime changing is not sane." Or should $] be the "current version" as determined by "use v.5.**.0;" for the compiling scope?

--
bulk88 ~ bulk88 at hotmail.com

[p5pRT]

From @jkeenan

On Fri Dec 05 14​:10​:18 2014, sprout wrote​:

On Fri Dec 05 12​:57​:57 2014, public@​khwilliamson.com wrote​:

On 12/05/2014 01​:41 PM, Father Chrysostomos via RT wrote​:

On Fri Dec 05 12​:33​:28 2014, public@​khwilliamson.com wrote​:

This is a bug report for perl from khw@​khw.(none),
generated with the help of perlbug 1.40 running under perl 5.21.7.

-----------------------------------------------------------------
I noticed that expressions that contain read-only variables such
as
$^V
or $], etc don't seem to get constant-folded, so for example

if($] < 5.007) {
FOO()
} elsif ($] >= 5.007003) {
BAR()
} else {
BAZ()
}

should just have generated unconditional code that calls BAR(),
with
everything else gone.

I was surprised to find this, because I thought the optimizer
handled
such things, and this seems like a blatant oversight.

$ perl -le 'local $]; $]=3; print $]'
3

That kind of thing can be useful for testing porpoises (and
dolphins).

But if you do that, your local $] isn't marked read-only, so when a
variable is read-only, it could be constant-folded at compile time

But code already compiled will be bound to *] glob and see the
localisation change.

I’m not saying we shouldn’t necessarily optimise here; I’m just afraid
of the aftermath. After all, simpler, more ‘internal’ changes I make
seem to break more CPAN modules than I would like.

$] is not deprecated, but its documentation implicitly discourages its use​:

#####
$] See "$^V" for a more modern representation of the Perl version that allows accurate string comparisons.
#####

Perhaps this is a case where we can let sleeping dogs lie.

Thank you very much.
--
James E Keenan (jkeenan@​cpan.org)

[p5pRT]

From @rjbs

* James E Keenan via RT <perlbug-followup@​perl.org> [2014-12-06T09​:29​:23]

$] is not deprecated, but its documentation implicitly discourages its use​:

#####
$] See "$^V" for a more modern representation of the Perl version that
allows accurate string comparisons.
#####

Perhaps this is a case where we can let sleeping dogs lie.

The same questions, related to treating the version as an immutable constant,
apply to $^V.

--
rjbs

[p5pRT]

From @khwilliamson

On 12/06/2014 08​:37 AM, Ricardo Signes wrote​:

* James E Keenan via RT <perlbug-followup@​perl.org> [2014-12-06T09​:29​:23]

$] is not deprecated, but its documentation implicitly discourages its use​:

#####
$] See "$^V" for a more modern representation of the Perl version that
allows accurate string comparisons.
#####

Perhaps this is a case where we can let sleeping dogs lie.

The same questions, related to treating the version as an immutable constant,
apply to $^V.

As an aside, we will never be able to get rid of $], because $^V is not
available in all versions of Perl. That means that modules that have to
work on very old Perls and have to distinguish between those old
versions are stuck with $]. Perhaps this should be spelled out in the
pods. A module for which all versions before v5.6 (when $^V became
available) are treated alike should use (! defined $^V || $^V lt foo)

[p5pRT]

From @mauke

Am 06.12.2014 um 18​:58 schrieb Karl Williamson​:

As an aside, we will never be able to get rid of $], because $^V is not
available in all versions of Perl. That means that modules that have to
work on very old Perls and have to distinguish between those old
versions are stuck with $]. Perhaps this should be spelled out in the
pods. A module for which all versions before v5.6 (when $^V became
available) are treated alike should use (! defined $^V || $^V lt foo)

I just wanted to add that this isn't necessary if your module uses
'warnings' or 'our', because those don't work pre-5.6 anyway.

--
Lukas Mai <plokinom@​gmail.com>

[p5pRT]

From @khwilliamson

On 12/06/2014 01​:15 PM, Lukas Mai wrote​:

Am 06.12.2014 um 18​:58 schrieb Karl Williamson​:

As an aside, we will never be able to get rid of $], because $^V is not
available in all versions of Perl. That means that modules that have to
work on very old Perls and have to distinguish between those old
versions are stuck with $]. Perhaps this should be spelled out in the
pods. A module for which all versions before v5.6 (when $^V became
available) are treated alike should use (! defined $^V || $^V lt foo)

I just wanted to add that this isn't necessary if your module uses
'warnings' or 'our', because those don't work pre-5.6 anyway.

If I understand you correctly, you're saying that if you use these
features, your code won't compile on pre-v5.6 anyway.

Also, I believe I was wrong. Correct me if I was right. According to
the docs, v-strings were introduced in v5.6, so any module that is
supposed to work in earlier versions can't use them outside of an eval
or else will get a syntax error. And that means $] is effectively
needed for such modules.

[p5pRT]

From @khwilliamson

On 12/06/2014 04​:59 PM, Karl Williamson wrote​:

On 12/06/2014 01​:15 PM, Lukas Mai wrote​:

Am 06.12.2014 um 18​:58 schrieb Karl Williamson​:

As an aside, we will never be able to get rid of $], because $^V is not
available in all versions of Perl. That means that modules that have to
work on very old Perls and have to distinguish between those old
versions are stuck with $]. Perhaps this should be spelled out in the
pods. A module for which all versions before v5.6 (when $^V became
available) are treated alike should use (! defined $^V || $^V lt foo)

I just wanted to add that this isn't necessary if your module uses
'warnings' or 'our', because those don't work pre-5.6 anyway.

If I understand you correctly, you're saying that if you use these
features, your code won't compile on pre-v5.6 anyway.

Also, I believe I was wrong. Correct me if I was right. According to
the docs, v-strings were introduced in v5.6, so any module that is
supposed to work in earlier versions can't use them outside of an eval
or else will get a syntax error. And that means $] is effectively
needed for such modules.

I managed to get perlbrew to install v5.5.4, and there is no syntax
error unless under 'strict'

[p5pRT]

From @Leont

On Sat, Dec 6, 2014 at 3​:29 PM, James E Keenan via RT <
perlbug-followup@​perl.org> wrote​:

$] is not deprecated, but its documentation implicitly discourages its use​:

#####
$] See "$^V" for a more modern representation of the Perl version
that allows accurate string comparisons.
#####

Perhaps this is a case where we can let sleeping dogs lie.

Quite franky, I have much better experiences with using $] than with $^V.
Not in the last place because the latter completely changed its behavior in
5.10.

Leon