[Chicago-talk] Which "if" structure is faster?

[Jay Strauss]

Thanks Jim,  I thought the if would be faster too.  I wasn't really trying
to be a speed demon but thought it was an interesting question.

Jay
----- Original Message -----
From: "Jim Thomason" <jthomasoniii at yahoo.com>
To: "Chicago.pm chatter" <chicago-talk at mail.pm.org>
Sent: Saturday, February 21, 2004 12:41 PM
Subject: Re: [Chicago-talk] Which "if" structure is faster?


> This actually caught my attention and I figured I'd
> give it a crack. I was quite surprised with the
> results.
>
> use Benchmark;
>
> timethese(500000, {
>   'if' => sub {
>     my $x = 5;
>     if ($x - 5 != 0) {
>       foreach my $idx (1..100) {
>         my $foo = $idx ** 2;
>         $foo -= 17;
>       }
>     } else {
>       #who cares?
>     }
>   },
>   'else' => sub {
>     my $x = 5;
>     if ($x - 5 == 0) {
>       #who cares?
>     } else {
>       foreach my $idx (1..100) {
>         my $foo = $idx ** 2;
>         $foo -= 17;
>       }
>     }
>   }
> });
>
> Benchmark: timing 500000 iterations of else, if...
>   else:  2 wallclock secs ( 0.70 usr +  0.10 sys =
> 0.80 CPU) @ 625000.00/s (n=500000)
>   if:  7 wallclock secs ( 1.96 usr +  0.00 sys =  1.96
> CPU) @ 255102.04/s (n=500000)
>
> Naturally, I have no clue what your code is like, so I
> just inserted a time waster.
>
> As you can see, the else clobbers the if, running
> slightly more than twice as fast. I would've expected
> the if to be slightly faster, just assuming that the
> 'else' would have to jump in the code but the 'if'
> would have the code in place (I have no basis for that
> assumption, just seemed logical to me). Not knowing
> squat about the perl internals, I can't even
> hypothesize as to why the else is so much faster.
> Perhaps my test was flawed in some manner I'm not
> seeing.
>
> Regardless, also note that the difference is
> 255,000/sec vs. 625,000/sec. Realistically, I'd say
> this "optimization" will be utterly completely totally
> negligible. If you're trying to squeeze out
> performance, look elsewhere.
>
> Result? Write whichever one looks more clear in the
> code. Maintainability would be the issue here, not
> speed. Also, you might want to consider moving the
> "first time" check out of the loop and doing it in
> advance. If that's your only special case, you could
> step through it and do it, and then begin your
> infinite loop w/o the special case. Assuming your code
> would allow you to break it out of the loop, of
> course.
>
> -Jim.....
>
> --- Jay Strauss <me at heyjay.com> wrote:
> > Hi,
> >
> > If I have some one time processing, then a endless
> > loop (till the proc is
> > killed).  Which is faster or are they the same:
> >
> > if ($firstTime) {
> >     ...stuff...
> >     $firstTime = 0;
> > }
> > else {
> >     ...stuff...
> > }
> >
> >
> > or
> >
> >
> > if (! $firstTime) {
> >     ...stuff...
> > }
> > else {
> >     ...stuff...
> >     $firstTime = 0;
> > }
> >
> >
> > Just curious
> > Jay
> >
> > _______________________________________________
> > Chicago-talk mailing list
> > Chicago-talk at mail.pm.org
> > http://mail.pm.org/mailman/listinfo/chicago-talk
>
>
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