Pointer Tagging Techniques

In this lesson, you will learn how to store metadata directly inside unused pointer bits to create memory-efficient data structures.

The Principle of Pointer Tagging

On modern 64-bit architectures, memory addresses are rarely 64 bits wide. Due to alignment requirements (e.g., structs aligned to 8 or 16 bytes), the lower 3–4 bits of any valid heap-allocated pointer are always zero. Pointer tagging involves repurposing these "trailing" bits to store small flags or integers, effectively packing metadata into the pointer itself without allocating extra space.

Implementation Strategy

To store data in a pointer, you must treat the pointer as a usize (or a raw integer) using as casts, perform your bitwise operations, and then transmute it back. Because this involves potentially invalidating the pointer, it must be done within an unsafe block.

fn tag_pointer<T>(ptr: *mut T, tag: usize) -> *mut T {
    // Assert that the tag fits in the bottom 3 bits (for 8-byte alignment)
    assert!(tag < 8);
    // Combine the pointer address with the tag
    (ptr as usize | tag) as *mut T
}

fn get_tag<T>(tagged_ptr: *mut T) -> usize {
    // Mask out all but the bottom 3 bits
    (tagged_ptr as usize) & 0x7
}

fn get_ptr<T>(tagged_ptr: *mut T) -> *mut T {
    // Clear the bottom 3 bits to retrieve the original address
    ((tagged_ptr as usize) & !0x7) as *mut T
}

Critical Considerations

When using this technique, you must be extremely disciplined:

1. Alignment: You are only allowed to tag bits that are guaranteed to be zero. If your object alignment is only 4 bytes, you only have 2 bits (0, 1) available. If you try to tag a bit that holds part of an address, you will create an invalid pointer, leading to immediate segmentation faults upon dereference.
2. Dereferencing: Never dereference a tagged pointer directly. You must use get_ptr to strip the tags first. The Rust compiler expects pointers to point to valid, aligned memory; passing a tagged pointer to any safe function (or even standard library functions like Box::from_raw) is undefined behavior.
3. Architecture: This technique is inherently non-portable. You must ensure the target hardware provides the expected padding in the pointer representation.

Key Takeaway: Pointer tagging exploits memory alignment to store metadata in otherwise wasted bits, but it requires strict manual bit-masking and stripping before the pointer can be safely dereferenced.