GC-VM interface

The interface between garbage collector and virtual machine is bidirectional:

• gc.h contains functions, which GC exports for use in VM.
• vm_gc.h contains VM functions which are available to GC during its operation.

A number of conventions exist, which are not easily expressed as C++ header files. These include:

• Thread local nurseries, see gc_supports_frontier_allocation().
• Read and Write barrier interface, gc_heap_write_ref() and others.

The conceptual overview of the interface is given in GC Writers' guide

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