in reply to Re^2: quick sort. How do it faster
in thread quick sort. How do it faster

Hmm... I'm surprised that introducing that temporary variable for swaps makes such a difference, but it does appear to. Do you have any explanation? Is it because it's saving a couple of array indexing operations, or is it just that ($x,$y)=($y,$x) is implemented suboptimally by Perl?

use Moops; class Cow :rw { has name => (default => 'Ermintrude') }; say Cow->new->name

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Re^4: quick sort. How do it faster
by BrowserUk (Patriarch) on Oct 23, 2013 at 16:31 UTC
    I'm surprised that introducing that temporary variable for swaps makes such a difference, but it does appear to. Do you have any explanation?

    Not really. Just that it seems to be some inefficiency inherent in the way list ops are implemented in perl.

    Perhaps even more surprising is that -- for numeric data -- the old saw, triple xor-assignment is more efficient than both the list assignment and the introduced temporary:

    cmpthese -1,{
    a=>q[ my( $a, $b    ) = ( 1, 2 ); ( $a, $b ) = ( $b, $a )   for 1.
+.1000; ],
    b=>q[ my( $a, $b,$t ) = ( 1, 2 ); $t = $a, $a = $b, $b = $t for 1.
+.1000; ],
    c=>q[ my( $a, $b    ) = ( 1, 2 ); $a ^= $b ^= $a ^= $b      for 1.
+.1000; ]
};;
    Rate    a    b    c
a 3253/s   -- -34% -42%
b 4913/s  51%   -- -13%
c 5632/s  73%  15%   --

    [download]

    That really ought not be the case.

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Re^4: quick sort. How do it faster
by LanX (Saint) on Oct 24, 2013 at 12:41 UTC
    > is implemented suboptimally by Perl?

    I doubt that list assignments can be implemented more efficiently w/o JIT magic.

    The behaviour of caching all values before assigning them is well defined, e.g. in Learning Perl ch. 3.4:

    Since the list is built up before the assignment starts, this makes it easy to swap two variables' values in Perl

    OTOH one could have tied vars on LHS which are not side effect free, such that compatibility would break after optimization to a ring-replacement.

    IMHO this can only be resolved at runtime .. (or with complicated precompiled case checks)

    Cheers Rolf

    ( addicted to the Perl Programming Language)

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