Why does decrement loop runs faster than increment loop?

Question

I was going through increment/decrement operators, and I encountered that if i run a loop in decremental form in that case it will run faster than the same loop in incremental form. I was expecting that both will take equal time since same amount of steps will be followed. I searched through the web but could not find convincing answer to this. Is it due to the case that decrement operator takes less time as compared to increment operator?

for(int i = 100000; i > 0; i--) {}
for(int i = 1; i < 100001; i++) {}

A long time ago, i>0 was a slightly faster test than comparisons with non null values but I seriously doubt this optimization is still relevant. — Denys Séguret, May 10 '13 at 06:04
See an existing question and answers: http://stackoverflow.com/questions/8968785/which-loop-has-better-performance-increment-or-decrement/8968842#8968842 — Jeremy Unruh, May 10 '13 at 06:07
I see no evidence that it makes any difference. In both the cases above the loop would be optimised away to nothing. — Peter Lawrey, May 10 '13 at 06:34

Evgeniy Dorofeev · Accepted Answer · 2013-05-10T06:25:11.207

44

This is because in bytecode comparison with 0 is a different operation than comparison with a non-zero number. Actually i < 10001 requires to first load the number on stack then execute comparison, while i > 0 is executed as one operation. Of course there will be no speed difference in most cases because of JVM optimizations. But we can try to make it visible by running the code with -Xint option (interpreted mode execution only).

edited May 10 '13 at 06:25

answered May 10 '13 at 06:05

Evgeniy Dorofeev

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To verify that, change the loop to `for(int i = -100000; i < 0; i++) {}`. That eliminates this effect and reduces the difference to increment vs decrement (which should not make a difference). – Thilo May 10 '13 at 06:06
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@StinePike: CPU has to load 2 Operands to compare 2 numbers. There are special instruction to compare with 0, so only one operand must be loaded into a register. – jlordo May 10 '13 at 06:16
@StinePike I added some clarification to my answer – Evgeniy Dorofeev May 10 '13 at 06:21
Thanks .. that was nice of your – stinepike May 10 '13 at 06:21
@Evgeniy Dorofeev Excellent explaination sir .. +1 for you. – Nikhil Agrawal May 10 '13 at 06:23
what about the another answer :o .. it is showing opposite.. – stinepike May 10 '13 at 06:25
actually it's eaiser to see and compare the bytecode, I use Eclipse Bytecode plugin for that, it's easy – Evgeniy Dorofeev May 10 '13 at 06:29
you can also validate Evgeniy's answer by comparing the following. Performance will be same in following case. for(int i = 100000; i > 1; i--) {} for(int i = 2; i < 100001; i++) {} – Jack May 10 '13 at 07:25

Nikhil Agrawal · Answer 2 · 2013-05-10T06:36:55.083

Piyush Bhardwaj

I tested both the loops in online compiler but my increment loop is executing faster then decrement loop.

Program execution depends upon many factors. When sometimes we run the same program on same machine many times we gets different execution times. So it depends upon many factors.

See the results

for(int i = 1; i < 100001; i++) {

}

Increment loop -- http://ideone.com/irdY0e

for(int i = 100000; i > 0; i--) {

}

Decrement loop -- http://ideone.com/yDO9Jf

Sir Evgeniy Dorofeev has given an excellent explanation which an expert only can give.

Finally, you need to consider the performance of your CPU. When considering a benchmark to determine the overall performance of a Java application, bear in mind that bytecode execution, native code execution, and graphics each play a role. Their impact varies depending on the nature of the specific application.

Why does decrement loop runs faster than increment loop?

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