Can the JVM optimize multiple reads from an array

Question

Can the JVM optimize array reads when reading the same index multiple times? Consider the following:

public static void foo(int[] array) {
   for (int i=0; i<array.length; i++) {
       int value1 = array[i];
       ...
       int value2 = array[i];
       ...
    }
    ...
}

array[i] is read twice in the loop. Assuming array[i] is not reassigned in the loop, is the JVM allowed to assume that array[i] did not change and thus read its value just once? Since array is a passed-in mutable object, it could conceivably have changed between the first and second read.

I've looked the at the generated byte code and it does indeed read array[i] twice (daload). Is the JVM allowed to optimize this into one read?

It's really a question about the Java memory model. The JVM vendor is irrelevant. In reality it will be Sun's Hotspot JVM. — Steve Kuo
– Steve Kuo, Commented Feb 26, 2013 at 23:28

irreputable · Accepted Answer · 2013-02-26 23:22:15Z

2

Yes, the optimizer only considers single thread data flow, it doesn't care if a variable is changed by another thread, unless volatile/synchronized are involved.

Even if there's no optimization, the 2nd read should be really fast, around 1ns, since the data is most likely in L1 cache.

answered Feb 26, 2013 at 23:22

irreputable

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RudolphEst · Accepted Answer · 2013-02-26 23:22:19Z

0

I do not think the JVM will optimize this read, but the overhead of the daload operation should be insignificant.

If you want to read the value from the array only once, try assigning to a local variable (costing some trivial local variable stack memory)

int origVal = array[i];
int value1 = origVal;
...
int value2 = origVal;

Tests should prove that the change is arbitrary.

answered Feb 26, 2013 at 23:22

RudolphEst

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