amusing CoW string dedupe example

amusing CoW string dedupe example

[Eric Wong]

I've always known hash keys get deduplicated in Perl to save RAM
for a while; but it turns out that it's possible to (ab)use them
for dynamic strings even when the hash doesn't live beyond a
subroutine scope.

Looks like we'll be able to save RAM in some places :D

The following script takes between 5-9M of RAM depending on the
version of Perl I'm using (tested down to 5.16.3), but nearly 1G
if the early return is uncommented in the `dedupe' sub:

----8<----
#!perl -w
use strict;
use Devel::Peek;
sub dedupe {
	my ($k) = @_;
#return $k; # uncomment to disable CoW dedupe, needs ~1G of RAM
	my %cow = ( $k => undef );
	((keys %cow)[0]);
}

my $n = 3_600_000;
my $k = pack('S*',($n + 1)..($n + 50000)); # uint16_t array[50000]
my @x = map { dedupe($k) } (1..10000);

{
	no warnings 'once';
	$Devel::Peek::pv_limit = 10;
}
Dump(\@x);
print 'length: ',length($k), "\n";