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in reply to Re^3: Is undef list assignment a no-op?
in thread Is undef list assignment a no-op?

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%       --

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Re^5: Is undef list assignment a no-op?
by BrowserUk (Patriarch) on Mar 25, 2011 at 15:39 UTC

    Now my results look much like Tux's: 

    C:\test>junk74
       Rate      c      a      b
c    2566/s     --  -100%  -100%
a 1761856/s 68575%     --   -18%
b 2158325/s 84029%    23%     --

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    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
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Re^5: Is undef list assignment a no-op?
by repellent (Priest) on Mar 26, 2011 at 04:46 UTC
    Thanks for the benchmark! (Same thanks goes to Tux and BrowserUk).

    Hmm. This is what I got. I changed to using subs instead:
    use Benchmark qw(cmpthese);

my $a1='a'x9999999;
my $b1=12345;
cmpthese -1,{
    und=> sub { sub{ my( undef, $x) = @_ }->( $a1, $b1 ) },
    skp=> sub { sub{ my $x = $_[1]        }->( $a1, $b1 ) },
    var=> sub { sub{ my( $unused, $x ) = @_;}->( $a1, $b1 ) },
};

__END__
        Rate      var      und      skp
var   50.2/s       --    -100%    -100%
und 516222/s 1027915%       --     -32%
skp 762291/s 1517942%      48%       --

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