D433 - Guest thread-local storage is variant-II, self-pointer at the thread pointer

Status: decided Date: 2026-09-01

A guest thread's TLS block places its initialised data at the bottom, its zero-initialised data above that, and a self-referential pointer at the thread pointer itself, so a guest reading its own thread pointer gets the pointer back.

Why: This is the variant-II layout the guest's own compiled code expects when it dereferences its thread pointer to find its TLS block; a guest's initialised-data image is copied from where the loader already placed it, so no separate copy of that data is kept.

Rejected: a variant-I-style indirect layout - does not match what a thread-relative access compiled for this convention expects.