http://www.perlmonks.org?node_id=388976

[fall through switch/case in perl brought about mention of Duff's Device. It turns out this construct is portable to Perl from c. Here's an old message to FWP about it.]

Duff's Device is an insane optimization originally invented to speed up writes to a serial device. The original is in c, but I wanted to see if I can code it in Perl.

It actually works, with a little kneading and much less of the original 50% performance gain. This is to be expected, because we're emulating pointer arithmetic for (not very) random string access. Another slowdown is the inavailibility of number-only labels. Anyone care to improve on Gaal's Gobbeldygook? <g>

[the assignment to $to in Perl is spurious, of course. Replace it with your particular write_one_char_to_device() function]

use integer; [...] sub trivial_loop { my $from = shift; my $to; my $i; my $count = length $from; for ($i = 0; $i < $count; $i++) { $to = substr $from, $i, 1 } } sub duff { my $from = shift; # real life would use reference here, this is a + demo my $to; # dummy: simulate write to serial i/o port my $i = 0; my ($n, $count); $count = length $from; $n = ($count + 7) / 8; # use integer in effect goto l. ($count % 8); # number-only labels don't work :- +( l0: do { $to = (substr $from, $i++, 1); l7: $to = (substr $from, $i++, 1); l6: $to = (substr $from, $i++, 1); l5: $to = (substr $from, $i++, 1); l4: $to = (substr $from, $i++, 1); l3: $to = (substr $from, $i++, 1); l2: $to = (substr $from, $i++, 1); l1: $to = (substr $from, $i++, 1); } while (--$n>0); }

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Re: Duff's Device in Perl
by simonm (Vicar) on Sep 07, 2004 at 17:49 UTC
      For the record, the above snippet is from March 2000.

      Also for the record, it runs under warnings and use strict ;)