comment on

As I understand it, passing an argument to a subroutine allows you to work on the alias within that sub, without copying the value, eg:


   sub reduce_whitespace { $_[0] =~ s/\s+/ /g }
   $text = 'black    cat';
   reduce_whitespace($text);
   print $text;

   > black cat
[download]

so the size of the scalar shouldn't matter (unless you copy it : my $string = $_[0])

But, this doesn't seem to be the case. Look at my benchmark below, which compares using an alias with pass-by-reference. Referencing and dereferencing has a cost, so I would expect it to be slower. However, as the string grows in length, the ref version wins over the alias version:

Benchmark code

use Benchmark qw(cmpthese);
use strict;
use warnings;

my $original;
$original.= chr(int(rand(128))) for 1..1000;

my $add = $original;

for (1..20) {

    print "\n","Length of string: ",length($original),"\n";

    cmpthese (1000000,{
        ref =>      sub { my $new = $original; by_ref(\$new)  },
        alias =>    sub { my $new = $original; by_alias($new) },

    });
    $original.=$add;
}

sub by_alias {
   $_[0]  =~ s/\s+//;
}

sub by_ref {
   ${ $_[0] } =~ s/\s+//;
}
[download]

Results


Length of string: 1000
              Rate alias      ref   
alias    1098901/s    --      36%
ref       806452/s  -27%       --

Length of string: 2000
             Rate alias      ref   
alias    909091/s    --      20%
ref      757576/s  -17%       --

Length of string: 3000
             Rate alias      ref   
alias    751880/s    --       4%
ref      724638/s   -4%       --

Length of string: 4000
             Rate alias      ref
alias    653595/s    --      -5%
ref      689655/s    6%       --

Length of string: 5000
             Rate alias      ref
alias    571429/s    --     -12%
ref      649351/s   14%       --

Length of string: 6000
             Rate alias      ref
alias    497512/s    --     -18%
ref      606061/s   22%       --

Length of string: 7000
             Rate alias      ref
alias    450450/s    --     -23%
ref      581395/s   29%       --

Length of string: 8000
             Rate alias      ref
alias    409836/s    --     -23%
ref      534759/s   30%       --

Length of string: 9000
             Rate alias      ref
alias    369004/s    --     -23%
ref      476190/s   29%       --

Length of string: 10000
             Rate alias      ref
alias    331126/s    --     -21%
ref      420168/s   27%       --

Length of string: 11000
             Rate alias      ref
alias    299401/s    --     -22%
ref      381679/s   27%       --

Length of string: 12000
             Rate alias      ref
alias    275482/s    --     -22%
ref      353357/s   28%       --

Length of string: 13000
             Rate alias      ref
alias    254453/s    --     -22%
ref      325733/s   28%       --

Length of string: 14000
             Rate alias      ref
alias    233645/s    --     -23%
ref      304878/s   30%       --

Length of string: 15000
             Rate alias      ref
alias    210526/s    --     -25%
ref      282486/s   34%       --

Length of string: 16000
             Rate alias      ref
alias    192678/s    --     -27%
ref      265252/s   38%       --

Length of string: 17000
             Rate alias      ref
alias    181488/s    --     -29%
ref      255754/s   41%       --

Length of string: 18000
             Rate alias      ref
alias    169779/s    --     -31%
ref      245098/s   44%       --

Length of string: 19000
             Rate alias      ref
alias    158983/s    --     -32%
ref      233100/s   47%       --

Length of string: 20000
             Rate alias      ref
alias    151515/s    --     -34%
ref      228833/s   51%       --
[download]

This is on Perl 5.8.8 on x86_64

Clint

Update Reformatted the results so that they are always presented in the same order, not in order of speed

In reply to Pass by ref vs alias : why does scalar size matter? by clinton

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