On my machine (Debian Lenny, Perl 5.10.0 as above) the sequencing and clustering of results is still the same even if our variables are used. In truth the only thing that is changing is the specific values being passed in. There is no fundamental difference in the two benchmarks.

As regards benchmark results, with our variables, the only difference is that the gap between und/var on one hand and $x=$_[1] on the other widens considerably. That is hardly surprising though. ($unused, $x) is copying a 1 million character array into $unused. $x=$_[1] is not.

The result does vividly illustrate that undef in a list is not a no-op. If (undef, $x) = @_ were a no-op its performance should cluster with $x=$_[1], but it doesn't. It acts like ($unused, $x) which copies a very big string. However, that clustering pattern is also identical to the results tux and I got, albeit less dramatically. With either benchmark, if you want to use lists on the left side of an assignment from an array, then you are still no better off using $unused instead of undef.

Are you seeing something different on your machine?

use Benchmark qw(cmpthese);

our $a1='a'x1e6;
our $b1=12345;
cmpthese -1,{
    und=>q[ sub{ my( undef, $x) = @_ }->( $a1, $b1 ) ],
    skp=>q[ sub{ my $x = $_[1]        }->( $a1, $b1 ) ],
    var=>q[ sub{ my( $unused, $x ) = @_;}->( $a1, $b1 ) ],
};
#output
        Rate      var      und      skp
var   66.3/s       --     -10%    -100%
und   73.5/s      11%       --    -100%
skp 945230/s 1424595% 1286780%       --

[download]

In reply to Re^4: Is undef list assignment a no-op? by ELISHEVA
in thread Is undef list assignment a no-op? by repellent

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.