Re^2: Efficient bit-twiddling in Perl.

[Tux]

When you reduce the overhead of the function calls (by doing the work you actually time in a tight loop), you might get other numbers

#!/pro/bin/perl -slw

use 5.016;
use warnings;

use Benchmark qw{cmpthese :hireswallclock};

my $n = 0x80061861;

say "and_rshift:    $n => ", join (' : ', and_rshift    ());
say "lshift_rshift: $n => ", join (' : ', lshift_rshift ());
say "mixed:         $n => ", join (' : ', mixed         ());
say "mixed_assign:  $n => ", join (' : ', mixed_assign  ());
say "just_shift:    $n => ", join (' : ', just_shift    ());

cmpthese (-10, {
    and_rshift    => \&and_rshift,
    lshift_rshift => \&lshift_rshift,
    mixed         => \&mixed,
    mixed_assign  => \&mixed_assign,
    just_shift    => \&just_shift,
    });

sub and_rshift
{
    my ($top14, $nxt6, $mid6, $bot6);

    for (1..10000) {
        $top14 = ($n & 0xfffc0000) >> 18;
        $nxt6  = ($n & 0x0003f000) >> 12;
        $mid6  = ($n & 0x00000fc0) >> 6;
        $bot6  = ($n & 0x0000003f);
        }

    return ($top14, $nxt6, $mid6, $bot6);
    }

sub lshift_rshift
{
    my ($top14, $nxt6, $mid6, $bot6);

    for (1..10000) {
        $top14 = $n >> 18;
        $nxt6  = $n << 46 >> 58;
        $mid6  = $n << 52 >> 58;
        $bot6  = $n << 58 >> 58;
        }

    return ($top14, $nxt6, $mid6, $bot6);
    }

sub mixed
{
    my ($top14, $nxt6, $mid6, $bot6);

    for (1..10000) {
        ($top14, $nxt6, $mid6, $bot6) =
            ($n >> 18, $n >> 12 & 0x3f, $n >> 6 & 0x3f, $n & 0x3f);
        }

    return ($top14, $nxt6, $mid6, $bot6);
    }

sub mixed_assign
{
    my ($top14, $nxt6, $mid6, $bot6);

    for (1..10000) {
        $top14 = $n >> 18;
        $nxt6  = $n >> 12 & 0x3f;
        $mid6  = $n >> 6 & 0x3f;
        $bot6  = $n & 0x3f;
        }

    return ($top14, $nxt6, $mid6, $bot6);
    }

sub just_shift
{
    my ($top14, $nxt6, $mid6, $bot6);

    for (1..10000) {
        ($top14, $nxt6, $mid6, $bot6) =
            ($n >> 18, $n << 46 >> 58, $n << 52 >> 58, $n << 58 >> 58)
+;
        }

    return ($top14, $nxt6, $mid6, $bot6);
    }

and_rshift:    2147883105 => 8193 : 33 : 33 : 33
lshift_rshift: 2147883105 => 8193 : 33 : 33 : 33
mixed:         2147883105 => 8193 : 33 : 33 : 33
mixed_assign:  2147883105 => 8193 : 33 : 33 : 33
just_shift:    2147883105 => 8193 : 33 : 33 : 33
               Rate just_shift       mixed mixed_assign and_rshift lsh
+ift_rshift
just_shift    247/s         --         -2%         -43%       -48%    
+      -52%
mixed         252/s         2%          --         -42%       -47%    
+      -52%
mixed_assign  434/s        75%         72%           --        -9%    
+      -16%
and_rshift    480/s        94%         90%          10%         --    
+       -8%
lshift_rshift 520/s       110%        106%          20%         8%    
+        --