Optimization test: -= faster than +=

[Pattt]

I made a performace test recently...

var a = 100;
for (var i:int=0;i<10000000;i++){
	a+=1
}
trace(getTimer());
//traces 410

and then...

var a = 100;
for (var i:int=0;i<10000000;i++){
	a-=-1
}
trace(getTimer());
//traces 280

Both are doing exactly the same thing, but in different ways. The -= method did it much faster.
Can it be true?

[neznein9]

I tested this a while ago with similar results.

[VacantPsalm]

Lawl, I just did something like that except it was num++ and num += 1; "+=" was a little quicker. One sec and I'll test that thing too.... Awesome, I already have the program up from testing other things. (I've been testing a lot of things since I just got this speed test code.)...

...Meh, they were about the same when I tried it. Test A = "+=" and Test B = "-="...

Test A: 119
Test B: 116

Test A: 114
Test B: 116

Test A: 114
Test B: 115

Tested them three times in a row since it's so fast. However, when I switch the "+= 1" with "++" I got this...

Test A: 197
Test B: 127

Test A: 201
Test B: 120

Test A: 209
Test B: 181

Not always a huge difference, but still faster. I should change all my for loops to += 1 instead of ++...

...This post is probably going to sound so odd because I keep trying stuff before submitting it.:P Anyways, when I tried different ways to do the for loop they all were about the same for some reason.:? Maybe CS4 does things a bit differently.

[moagrius]

- is faster than +
--i is faster than i--
and since you're on performance, "for" is about 3x to 4x slower than "while" (that's for JS in all major browsers, and AS2... haven't tested in AS3 yet)... the tests below are a few years old.

flash player 8				javascript (IE6)
for	while	dowhile		for	while	dowhile
2563	1078	1093		672	156	171
3344	500	515		672	172	172
657	500	562		687	187	172
3609	469	531		688	172	172
797	515	547		687	172	187
687	500	547		672	188	156
3500	516	531		687	157	172
922	500	531		672	188	188
766	1000	484		687	171	172
3235	484	1093
859	453	515

averages
2093.9	651.5	694.9		612.4	156.3	156.2

[Pattt]

I posted this on Kirupa also, and someone pointed out that if you say "var a:Number", the difference will disappear.

[cancerinform]

var a:int = 100;
for (var i:int=0; i<10000000; i++)

119

var a:uint = 100;
for (var i:uint=0; i<10000000; i++)

537

var a:Number = 100;
for (var i:Number=0; i<10000000; i++)

1243

var a = 100;
for (var i=0; i<10000000; i++)

2785

I repeated in all cases and got similar results. int is fastest, followed by uint, followed by Number followed by none.

[VacantPsalm]

Just tested the for vs while thing. I think they fixed that for AS3. Here's my code, in case I'm doing something wrong...

import flash.utils.getTimer;
var time:Number = getTimer();

function TestA():void
{
	var i:int = 0;
	time = getTimer();
	while (i<100000000)
	{
		i += 1;
	}
	trace("Test A: ", (getTimer()-time));
}

function TestB():void
{
	time=getTimer();
	for (var i:int=0; i<100000000; i += 1)
	{
	}
	trace("Test B: ", (getTimer()-time));
}
TestA();
TestB();

Tried that four times in a row, the outputs were...

Test A: 1173
Test B: 1080

Test A: 1125
Test B: 1121

Test A: 1116
Test B: 1109

Test A: 1261
Test B: 1105

I keep getting a weird fluke every once in a while where the while loop takes longer then the for loop. But overall I'd say they're about the same.

Edit: Oh yea, I forgot that I was doing 100,000,000 instead of 10,000,000. When I went back down to 10,000,000 the difference was like between 1-5 each time I ran it. A few results...

Test A: 109
Test B: 106

Test A: 109
Test B: 109

Test A: 106
Test B: 108

[neznein9]

For and while loops get compiled to the same bytecode in AS3 - the place you lose performance is in the increment and the conditional...so

//  slow - has to check if i is less than 10000000 every time
while (i<100000000){
    i += 1;
}


//  faster - just has to check that i is != 0
i = 100000000
while(i){
    i += -1;
}