Header Compression
Published on 2022-11-29. Last updated on 2025-07-12
Introduction
Consider a reference-based class definition class Cat(name: String, lives: Int64).
Then an instance Cat("Colin", 9) may be represented in memory like this:
+---------------+---------------+---------------+
| header | reference | 9 |
+---------------+---------------+---------------+
|
v
"Colin"
- The header needs to include information on the instance’s type for dynamic dispatch and garbage-collection purposes.
- The runtime may need a number of bits for miscellaneous uses.
- Some garbage-collection algorithms will also require a forwarding pointer, which might be placed in the header.
A smaller header is generally desirable as it improves memory efficiency and data locality, and decreases cache pressure.
Schemes to decrease header size often trade in a more compressed representation of information with additional computational effort.
Example Header Layout
- If vtable/type/class space can be allocated in the lower 4GB of virtual memory, then the vtable/type/class pointer only requires a shift to dereference the vtable/type/class.
- If vtable/type/class space cannot be allocated in the lower 4GB of virtual memory, then the vtable/type/class pointer requires a shift and an addition of a fixed offset to dereference the vtable/type/class
- Not all garbage-collection algorithms require a forwarding pointer (or a forwarding pointer placed in the header)
4-byte instance header
+---------------+---------------+---------------+---------------+
| ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ |
+---------------+---------------+---------------+---------------+
╰──────────────────────┬──────────────────────╯ ╰──────┬──────╯
24 bits: vtable/type/class pointer 8 bits: other uses
128 byte granularity, 4GiB address space (type 7)¹ .
8-byte instance header, with space for a forwarding pointer
+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+
| ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ | ┊ ┊ ┊ ┊ ┊ ┊ ┊ |
+---------------+---------------+---------------+---------------+---------------+---------------+---------------+---------------+
╰──────────────────────┬──────────────────────╯ ╰──────────────────────────────┬──────────────────────────────────────╯ ╰──┬──╯
24 bits: vtable/type/class pointer 36 bits: forwarding pointer 4 bits: other uses
128 byte granularity, 4GiB address space (type 7)¹ 8 byte granularity, 1TiB address space (type 4)²