Safepoint

A safepoint is a code location where the compiler's side tables (GCInfo, in NativeAOT) describe the GC state of a method exactly: which registers and stack slots hold live references at that point. Between safepoints the tables say nothing, so a precise scan is only sound for a thread stopped at one.

Call sites are the safepoints that matter for stack walking. While a method waits for a callee to return, its frame is frozen at the call instruction, so every return address on a thread's stack identifies a safepoint in its caller. A thread that entered the collector through a chain of calls can therefore be scanned precisely frame by frame, while a thread preempted at an arbitrary instruction cannot. Runtimes bridge that gap with return-address hijacking: force the preempted thread to run to a safepoint, then scan it.

In OrionGC only the collection-triggering thread is known to sit at safepoints; every other thread is scanned conservatively until hijacking exists (#385). See The safepoint constraint.