While getting to that point of a project where optimizing and reviewing my Perl chops begins to seem like a good idea, i decided to check on some assumptions I'd made. Well, ok, not assumptions, I was following what basically most articles on the topic of passing by reference or by copy seem to say, and which I'd, sadly, followed without verifying the murky depths of mythology myself.

This post helped me in my path, and of course, reminded me of the vast pool of things I don't know: Performance issues with subroutines/methods

So I decided to test it, sure in my faith, for faith is what it turns out to be, that of course, passing by reference must always be fast than passing copies, since, well, Perl doesn't have to work much to use a references, but it has todo all kinds of work when it creates copies.

So I tested it: 

 
use strict; 
use warnings; 
use Benchmark qw(:all); 

my @a = ( 'a','b','c','d','e','g','h', 'a','b','c','d','e','g','h', 'a
+','b','c','d','e','g','h', 'a','b','c','d','e','g','h', 'a','b','c','
+d','e','g','h', 'a','b','c','d','e','g','h', 'a','b','c','d','e','g',
+'h', 'a','b','c','d','e','g','h', 'a','b','c','d','e','g','h', 'a','b
+','c','d','e','g','h', ); 

my $a_ref = [ 'a','b','c','d','e','g','h', 'a','b','c','d','e','g','h'
+, 'a','b','c','d','e','g','h', 'a','b','c','d','e','g','h', 'a','b','
+c','d','e','g','h', 'a','b','c','d','e','g','h', 'a','b','c','d','e',
+'g','h', 'a','b','c','d','e','g','h', 'a','b','c','d','e','g','h', 'a
+','b','c','d','e','g','h', ]; 

cmpthese(-2, { 
  'a copy'  => sub { my @b = a_copy(@a); }, 
  'a ref 1' => sub { my $b = a_ref_1(\@a); }, 
  'a ref 2' => sub { my $b = a_ref_2(\@a); }, 
  'a ref 3' => sub { my $b = a_ref_1($a_ref); }, 
  'a ref 4' => sub { my $b = a_ref_2($a_ref); }, 
}); 

sub a_copy { my (@a) = @_; return @a; } 
sub a_ref_1 { my ($a) = @_; return $a; } 
sub a_ref_2 { return $_[0]; }

[download]


Which returned, much to my dismay, the following: 

 
             Rate a copy a ref 1 a ref 3 a ref 2 a ref 4 
a copy   171189/s    --    -97%    -97%    -97%    -98% 
a ref 1 5073751/s  2864%     --    -15%    -21%    -35% 
a ref 3 5946679/s  3374%    17%     --      -7%    -23% 
a ref 2 6408282/s  3643%    26%      8%     --     -17% 
a ref 4 7753141/s  4429%    53%     30%     21%     --

[download]


The larger I made the test arrays, the larger the gaps grew, if I made them 2x larger, they took 2x longer for references, roughly, it seemed failrly linear.

What am I missing here? I fear I have somehow totally missed some core, essential feature of Perl during this journey, which is, sadly, all too likely, but this really did not meet my expections or beliefs at all. Note that following the notion that references must always be faster, I actually converted a big chunk of a big project to always pass hashes and arrays to subs by reference, not by copying, and now I'm wincing that I did not test this first.

Note that in the above, I speculated that maybe Perl had to work extra hard to create a reference at every iteration using (\@a) so I also created a pre-referenced array, and passed the scalar directly in, and, again, much to my conceptual dismay, that was even slower!!! How can this be? I mean, this data clearly suggests that I should never pass back or forth by reference, so what am I missing here?

I realize during all spiritual journeys, we hit stumbling blocks, challenges, and so on, and that at times. we must consult with our gurus to obaz

Update:

Delete this if needed, I was just misreading, badly, some results, and the question has no relevance and is only a warning to others that sometimes it's good to step away from the keyboard for a few days when you start making mistakes this bad. My apologies to the monks.

In reply to A matter of sub pass by reference or copy by h2

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