[perl #114536] Optimize assigning to scalars from @_

# New Ticket Created by  David Golden 
# Please include the string:  [perl #114536]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org:443/rt3/Ticket/Display.html?id=114536 >


This is a bug report for perl from dagolden@cpan.org,
generated with the help of perlbug 1.39 running under perl 5.16.0.


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

This ticket suggests a possible peephole optimization, should someone
be interested in pursing it.  It is based on a discussion I had with
Nicholas Clark and I agreed to write it up so the idea wouldn't be
lost.

In short, a huge number of pure Perl subroutines start off like this:

  sub foo {
    my ($one, $two) = @_;
    ...
  }

This assignment requires multiple OPs to execute:

  $ perl -MO=Concise,foo,-exec -e 'sub foo {my ($one, $two) = @_; dump }'
  main::foo:
  1  <;> nextstate(main 1 -e:1) v
  2  <0> pushmark s
  3  <#> gv[*_] s
  4  <1> rv2av[t4] lK/1
  5  <0> pushmark sRM*/128
  6  <0> padsv[$one:1,2] lRM*/LVINTRO
  7  <0> padsv[$two:1,2] lRM*/LVINTRO
  8  <2> aassign[t5] vKS
  9  <;> nextstate(main 2 -e:1) v:{
  a  <0> dump s*
  b  <1> leavesub[1 ref] K/REFC,1

If this is predictable and detectable at the start of a subroutine --
albeit with a variable number of padsv's being assigned -- then it
should be possible for the peephole optimizier to replace them with a
new, single OP that copies an arbitrary number of SV's directly from the
stack to the pad.  Depending on the overhead of such a peephole check on
every sub, this could potentially speed up code with many small
subroutines such as occurs in highly-factored code.

(I don't know if similar optimization could be made to work -- or would
be worthwhile -- for shift or array/hash assignment.)

-- David


[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=core
    severity=wishlist
---
Site configuration information for perl 5.16.0:

Configured by david at Fri Jul 20 11:04:40 PDT 2012.

Summary of my perl5 (revision 5 version 16 subversion 0) configuration:

  Platform:
    osname=linux, osvers=3.0.0-22-generic, archname=x86_64-linux
    uname='linux icarus 3.0.0-22-generic #36-ubuntu smp tue jun 12 17:37:42
utc 2012 x86_64 x86_64 x86_64 gnulinux '
    config_args='-de -Dprefix=/home/david/perl5/perlbrew/perls/perl-5.16.0'
    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=undef
    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='-O2',
    cppflags='-fno-strict-aliasing -pipe -fstack-protector
-I/usr/local/include'
    ccversion='', gccversion='4.6.1', 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='double', nvsize=8, Off_t='off_t',
lseeksize=8
    alignbytes=8, 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 -ldl -lm -lcrypt -lutil -lc
    perllibs=-lnsl -ldl -lm -lcrypt -lutil -lc
    libc=, so=so, useshrplib=false, libperl=libperl.a
    gnulibc_version='2.13'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
    cccdlflags='-fPIC', lddlflags='-shared -O2 -L/usr/local/lib
-fstack-protector'

Locally applied patches:


---
@INC for perl 5.16.0:

/home/david/perl5/perlbrew/perls/perl-5.16.0/lib/site_perl/5.16.0/x86_64-linux
    /home/david/perl5/perlbrew/perls/perl-5.16.0/lib/site_perl/5.16.0
    /home/david/perl5/perlbrew/perls/perl-5.16.0/lib/5.16.0/x86_64-linux
    /home/david/perl5/perlbrew/perls/perl-5.16.0/lib/5.16.0
    .

---
Environment for perl 5.16.0:
    HOME=/home/david
    LANG=en_US.UTF-8
    LANGUAGE=en_US:en
    LC_COLLATE=C
    LC_CTYPE=en_US.UTF-8
    LC_MESSAGES=en_US.UTF-8
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)

PATH=/home/david/perl5/perlbrew/bin:/home/david/perl5/perlbrew/perls/perl-5.16.0/bin:~/bin:~/git/utility-scripts:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/opt/vagrant/bin:.
    PERLBREW_BASHRC_VERSION=0.45
    PERLBREW_HOME=/home/david/.perlbrew
    PERLBREW_MANPATH=/home/david/perl5/perlbrew/perls/perl-5.16.0/man

PERLBREW_PATH=/home/david/perl5/perlbrew/bin:/home/david/perl5/perlbrew/perls/perl-5.16.0/bin
    PERLBREW_PERL=perl-5.16.0
    PERLBREW_ROOT=/home/david/perl5/perlbrew
    PERLBREW_VERSION=0.45
    PERL_BADLANG (unset)
    PERL_EXTUTILS_AUTOINSTALL=--defaultdeps
    SHELL=/bin/bash

• [perl #114536] Optimize assigning to scalars from @_ by David Golden