D274 - Guest callbacks are called with the System V convention

Status: decided Date: 2026-08-25

An implementation that calls back into guest code, such as a sort comparator, declares the function pointer extern "sysv64" and calls it directly; a nested import call from the callback re-enters the dispatcher as an ordinary call. qsort sorts an index permutation and applies it afterwards, so the guest's array does not move while comparisons run.

Why: the compiler then emits the guest's own convention, the same machinery used to enter a guest, and signal handlers and callback-taking calls reuse it. Leaving the array in place is stricter than the standard requires and never looser.

Rejected:

• A hand-written call shim per callback: duplicates what the compiler emits.
• Sorting the guest's array in place during comparisons: a comparator that inspects neighbours sees elements move.