This will be a thread about optimization and efficiency concerns in both programming in general, and specifically to perl, if need be. Making algorithms more efficient is a much needed ability to any programmer, and there are some common tips/tricks/whatever that can help.

Personally, I know of a few, and wondered what else was out there--perl-related or not. Some of what I'll offer is this:

(1)The unrolling the loop technique (not Friedl's regex optimization). To explain, the first code example is more than likely slower than the second: 

for($i = 0; $i <= 10_000; $i++) { 
    $ary[$i] = init_val($i);
}

[download]

$idx = 0;
for($i = 0; $i < 10_000; $i += 10) {
    $ary[$idx++] = init_val($idx);
    $ary[$idx++] = init_val($idx);
    $ary[$idx++] = init_val($idx);
...rest goes here...
}

[download]

(2)Shifting for Multiplying and Dividing. It should come naturally that moving over some bits by a few positions is relatively fast for a computer based entirely on bits, and doing mathematical operations such as multiplying are about an order of a magnitude faster. For example:
$x = $y * 16;
$x = $y << 4; #Much faster

[download]

To get this to work for something besides a power of two what we do is use the fact that $y * 26 can be expressed as this:
      $y * 26 
   == $y * 16 + $y * 8 + $y * 2 
   == $y << 4 + $y << 3 + $y << 1;

[download]

Also, division is fater to be expressed not as $x / 6 but as $x * 1/6, which holds true for most general cases.
(3)Look-up tables can make for faster code
(4)sin(angle) = cos(angle-90) and cos(angle) = sin(angle+90) (this helps, I should note, in forming look-up tables of sine or cosine values; also, tan is redundant, so using just sin or just cos can make it faster)
(5)x % y can be expressed better as x & (y-1) (*big note* this is only for powers of 2, e.g. x%64==x&63
(6)Assembly is faster generally, if not always--if implemented correctly; though I'm not aware of how to get inline assembly code in perl, it's easy in C/C++ with _asm.

Update: To the rather bold question of whether I need to optimize: yes. Well, at least in my current project; I'm working on making a game, and parts of the game need to go real fast to make it look realistic. The game will be done in C++, and that's why I focussed the post on 'C-style constructs' and things that may not work too well in an interpreter language. I just posted on this site because (1)there isn't a C site with as good feedback, and (2)some things are general, and many perl programmers came, or have experience in, C or C++. I should have said something to avert this confusion, so I apologize; but I hope that explains myself.

I also fixed the for loop, and added some things that I should have put in the original post.

In reply to Optimizations and Efficiency by dimmesdale

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