Earlier today, in the CB, I talked with arturo about how to find out whether a given integer is even or odd.

The obvious way to do this involves something like: $odd = $num % 2;.

On the other hand, my CS professor claims that's a slow way of finding out the oddness of a number, and suggests something along the lines of: $odd = $num & 1; instead. This makes sense, because one would assume that a logical AND takes up less CPU time than a DIV instruction.

I decided to test this hypothesis using perl's Benchmark module, and was surprised at the results:

Here's the code: 

my $t1 = timeit(1_000_000, \&odd_1);
my $t2 = timeit(1_000_000, \&odd_2);
print "odd_1: ",timestr($t1),"\n";
print "odd_2: ",timestr($t2),"\n";

sub odd_1 {
  my $num = rand(2**31);
  return $num % 2;
}

sub odd_2 {
  my $num = rand(2**31);
  return $num & 1;
}

[download]
And here, the results: 
[falkkin@shadow ~/perl] perl odd.pl
odd_1:  6 wallclock secs ( 6.29 usr + -0.00 sys =  6.29 CPU) @ 158982.
+51/s (n=1000000)
odd_2: 10 wallclock secs ( 9.36 usr + -0.01 sys =  9.35 CPU) @ 106951.
+87/s (n=1000000)

[download]
It appears that the "slow" method of computing this is actually faster, but I have no idea why. I was wondering if any monks here who know the internals of perl could provide any insight on this.

The only causes I can think of are:
- My CS teacher's claim is simply wrong; she maybe didn't account for newer hardware that is optimized for multiplication & division (although this script is being run on a Pentium I with MMX instructions, which is hardly cutting-edge any more...)
- Perhaps perl's compiler optimizes $num % 2 into "return the last bit of $num"?

Thoughts, anyone?

In reply to Testing a number for oddness by Falkkin

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