Object and array inlining continued

Objects and array inlining operate on a group of objects and arrays in the heap that are in a parent-child relationship. These nest levels allow the reference of a child to be directly in the parent even if a parent has multiple children. There can be a hierarchy of such levels but the difference is only between object fields and array fields. 

The bytecode is the same between array and object fields but the inlining is different between the two. The size of the object is known beforehand. The size of an array is not known until allocation. Hot fields are worth more for optimizing. The Just-in-time compiler is leveraged for this purpose which inserts read barriers that increment field access counters per field and class. 

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 improvement can be seen this way. Let us say Parent and Child are two nested objects. Parent has a member c that points to a child and the child has a member f. Then the access p.c.f would cause the following instructions traditionally: 

eax: p 

mov ebx, [eax+8]  

mov ecx, [ebx+8] 

ecx: p.c.f 

Instead, the optimized machine code is now: 

eax: p 

mov ecx, [eax + 24] 

ecx: p.c.f  

The object and array inlining are different. 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.