Consider a Java based implementation of a Java interpreter that encodes the primitive fields of an object in a byte array. For example:

class MyClass {
    long l;
    int i;
    boolean b;
}

could be encoded as a byte array of length 13:

byte[] data = {
    l0, l1, l2, l3, l4, l5, l6, l7,
    i0, i1, i2, i3,
    b1
}

Implementing access to field i can be done with Unsafe:

setter: Unsafe.putInt(obj, Unsafe.ARRAY_BYTE_BASE_OFFSET + 8, val);
getter: Unsafe.getInt(obj, Unsafe.ARRAY_BYTE_BASE_OFFSET + 8);

A JVMCI compiler (such as Graal) may be able to escape analyze a MyClass object and represent it as registers and stack locations. If such a "virtualized" object is alive at a deoptimization point, the compiler would like to encode info for `data` that indicates how the fields are encoded in the byte array. This can be done in jdk.vm.ci.code.VirtualObject. A primitive value of a kind of n bytes in a byte array at index i results in a VirtualObject instance with its entry at i being the full primitive value, and the n-1 next entries being the illegal constant.
During deoptimization, materializing the entry at i involves counting the number of following illegals starting from i+1, and from that deducing how many bytes to write at once. The entry at i is then written over said illegals, and the illegals are skipped.
This allows to craft a compact representation of primitive fields that benefits from escape analysis.

As an example, the VirtualObject representing for a MyClass instance {l=42, i=7, b=true} would be:

{42, ILLEGAL, ILLEGAL, ILLEGAL, ILLEGAL, ILLEGAL, ILLEGAL, ILLEGAL, 7, ILLEGAL, ILLEGAL, ILLEGAL, true}