Declaring multiple arrays with 64 elements 1000 times faster than declaring array of 65 elements

Question

Recently I noticed declaring an array containing 64 elements is a lot faster (>1000 fold) than declaring the same type of array with 65 elements.

Here is the code I

Answer 1

There are any number of ways that there can be a difference, based on the size of an object.

As nosid stated, the JITC may be (most likely is) allocating small "local" objects on the stack, and the size cutoff for "small" arrays may be at 64 elements.

Allocating on the stack is significantly faster than allocating in heap, and, more to the point, stack does not need to be garbage collected, so GC overhead is greatly reduced. (And for this test case GC overhead is likely 80-90% of the total execution time.)

Further, once the value is stack-allocated the JITC can perform "dead code elimination", determine that the result of the new is never used anywhere, and, after assuring there are no side-effects that would be lost, eliminate the entire new operation, and then the (now empty) loop itself.

Even if the JITC does not do stack allocation, it's entirely possible for objects smaller than a certain size to be allocated in a heap differently (eg, from a different "space") than larger objects. (Normally this would not produce quite so dramatic timing differences, though.)