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Trash Talk - So, you wanna optomize your loop, huh?

CypherJF - Fri Apr 01, 2005 6:40 pm
Post subject: So, you wanna optomize your loop, huh?

An interesting article:

http://www-128.ibm.com/developerworks/power/library/pa-unrollav3/

I haven't had time to read it completely

Mr Ekted - Fri Apr 01, 2005 7:34 pm
Post subject:

Classic example (if you care about speed):

Code: Show/Hide

// bad code

for (i = 0; i < 1000000; i++)
{
if (something)
Func1(i);
else
Func2(i);
}

// good code

if (something)
for (i = 0; i < 1000000; i++)
Func1(i);
else
for (i = 0; i < 1000000; i++)
Func2(i);

Of course this is a hypothetical case. You would more likely embed the loops in the functions instead of calling them a million times.

Cyan~Fire - Fri Apr 01, 2005 11:55 pm
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That article wasn't really about optimizing loops, it was optimizing loops with AltiVec or whatever.

What Ekted said applies far more to the majority of us. (Those of us who don't use PowerPCs?)

Dr Brain - Sat Apr 02, 2005 12:15 am
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Shouldn't any decent optimizer fix that, ekted?

Besides, readability is far more important that efficiency in ~90% of cases.

Mr Ekted - Sat Apr 02, 2005 12:42 am
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I would agree in most cases, but if you are writing something that absolutely has to be killer fast--maybe you would even go so far as to profile it--then this is something you would seriously consider. A thousand loops, no big deal, A million, you are starting to get iffy.

Maverick - Sat Apr 02, 2005 8:47 am
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Mr Ekted wrote:

Classic example (if you care about speed):

Code: Show/Hide

Hm.. can you explain me that please? Why is running two identical loops better then only running one?

D1st0rt - Sat Apr 02, 2005 9:35 am
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It only has to do the conditional branch once.

Maverick - Sat Apr 02, 2005 9:41 am
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D1st0rt wrote:

It only has to do the conditional branch once.

The if() takes longer then the for loop?

D1st0rt - Sat Apr 02, 2005 9:42 am
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In the first example, it has to do the if 1000000 times. In the second, it does it once.

Maverick - Sat Apr 02, 2005 9:43 am
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aaaah... but two for loops of that size should still make up more cpu cycles then one (with the if() )? Or am I mistaken..

Cyan~Fire - Sat Apr 02, 2005 11:07 am
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It'll be executing more than one command in ASM, if that's what you mean. But in both cases, the for loop commands stay the same, but the conditional code only has to be done once in the second, which obviously makes it faster no matter how long the for()s take.

D1st0rt - Sat Apr 02, 2005 12:57 pm
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In the second example, only one of the loops gets run

SamHughes - Sat Apr 02, 2005 9:38 pm
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What should I do if looping through branches a million times is my favorite form of gambling?

Bak - Sat Apr 02, 2005 10:43 pm
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SamHughes wrote:

What should I do if looping through branches a million times is my favorite form of gambling?

end yourself

Maverick - Sun Apr 03, 2005 6:11 am
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SamHughes wrote:

What should I do if looping through branches a million times is my favorite form of gambling?

Try russion roulette as your final gambling game.