Object and array inlining continued

 Object and array inlining work best when the hot fields are detected. The Just-in-time compiler helps with this by inserting read barriers that increment field access counters per field and class. And the garbage collector pitches in by moving objects that are linked by hot fields into groups such that the parent and its children are consecutive.

The inlining works when the following two conditions are met. First, the parent and child objects must be allocated together and the field store that places the reference of the parent in the child must happen immediately afterwards so that it remains true for the lifetime of the data structures that are collocated this way. Second, the field stores must not overwrite the field with a new value. 

The leveraging of compiler and garbage collector for inlining is common to both object and array inlining bu there is also a difference. When arrays are used as inlining children, the size of arrays is not clear as a compile-time constant. The Java bytecodes for accessing array elements have no static type information. The preconditions can be made similar to that of object inlining if the following three instructions are combined into a single instruction for collocation-  the object allocation of parent, the array allocation of the child, and the field store of the array field.

There are three types of inlining: fixed array inlining, variable array inlining, and dynamic array inlining.
Fixed array inlining is one where the array fields can be handled the same way as object fields because the length is constant.
Variable array inlining is one where the array fields have different but fixed lengths.
Dynamic array inlining is one where the array field is assigned multiple times.

Fixed Array inlining if all objects of a parent class reference arrays with the same fixed length, then the field access for the array fields can be eliminated because their offset is known. In addition, subsequent access can also be optimized by eliminating the array bounds check. There are two additional instructions required over object field access. The first one checks the index against the length and the second one branches to an out of line code block that throws an exception. Instead of three loads - only one load is necessary with the two eliminations.

Variable array inlining: The objects of a parent class reference arrays with different but fixed lengths. The parent can have only one variable array inlining child because the inlining offset of a subsequent child cannot be known with the variable length of the first. In this case the two memory loads are necessary.

Dynamic array inlining: When an array field is assigned multiple times, one more check needs to be included for the field access because it is not safe to eliminate it. However, unlike object field, it is possible to detect if an array field has been changed at runtime.