| Description: |
[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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