Non-modal bounds

Externality

The externality axis records whether all a type’s values may safely be ignored by the GC. This may be because they are OCaml “immediates” (values represented by a tagged integer), because they are unboxed types like float# or int32#, or because they are allocated elsewhere.

The axis has three possible values, with external_ < external64 < internal.

• external_ means that all values of the type are safely ignored by the GC.
• external64 means that all values of the type are safely ignored by the GC on 64-bit systems. The only 32-bit target currently supported by the OxCaml compiler is bytecode. Note that, although JavaScript and WASM are 32-bit platforms and the compiler goes through bytecode to reach them, they still count as 64-bit systems for the purpose of this axis because of their unique data models.
• internal means values of the type may need to be scanned.

The compiler uses the externality axis for certain runtime optimizations. In particular, updating a mutable reference to a type that is external_ can skip the write barrier (i.e., it does not need a call to caml_modify).

Writes of types with non-value (i.e. unboxed) base layouts never require a write barrier, as values of such types must never be seen by the garbage collector. However, since externality is tracked as a non-modal property of kinds, as opposed to part of its layout, it is possible to express types with layout e.g. float64 which are not external. This is very rarely desirable; it is typically far more convenient to preserve externality information than it is to enforce some semantic property of an unboxed type by hiding it. For this reason, the kinds bits8, bits16, bits32, bits64, float32, float64, untagged_immediate, vec128, vec256, vec512, void, and word all imply mod external_.

In the future, we plan to make externality a mode, rather than just a property of types.