Why does right shift perform better than divide in JAVA?

Asked Apr 21 '15 at 06:40

Active Apr 21 '15 at 07:01

Viewed 63 times

As we know, JIT will optimize division to have a better performance. But when I try to calculate time that division and right shift cost:

long j=0;
long a1=Calendar.getInstance().getTimeInMillis();
for (long i = 0; i < 999999999l; i++) {
        j=i/2;//or j=i>>1;
}
long a2=Calendar.getInstance().getTimeInMillis();
System.out.println(a2-a1);

Result: j=i/2; 1070ms j=i>>1; 702ms

I try it on different platforms but the results are quite same: right shift performs better. Why doesn't JVM or JIT turn the /2 into >>1 to get better performance?

Thanks all. It seems that if dividend is a variable, compiler will not do the optimization. I change the dividend into a constant and the result shows /2 and >>1 perform equivalent. PS. Thanks, System.currentTimeMillis() is better.

edited Apr 21 '15 at 07:01

asked Apr 21 '15 at 06:40

F Yl

6

They’re not the same operation for negative numbers. – Ry- Apr 21 '15 at 06:42
Some compilers are able to optimize out the loop entirely by removing it. – Basile Starynkevitch Apr 21 '15 at 06:42
@minitech that should be an answer – fge Apr 21 '15 at 06:43
by the way, use `System.currentTimeMillis()` or `System.nanoTime()` for timing – alex Apr 21 '15 at 06:48
By the way, the bytecodes for both operations are not the same; integer divide is `idiv` and right shift is `ishr` – fge Apr 21 '15 at 06:51
in decimal do you find it easier to shift 10000 by 2 digits or to divide 2 numbers? – phuclv Apr 21 '15 at 08:13

Why does right shift perform better than divide in JAVA?

0 Answers0