Currently the nursery is made up a of series of chunks which are not contiguous in memory. Reserving address space ahead of time and using this to allocate nursery memory contiguously could be better for the following reasons:

• cheaper post barriers (which don't require loads from the chunk header)
• simplification of nursery code
• it's a prerequisite for pointer compression

Possible downsides are:

• increased use of address space (may negatively affect 32bit platforms)

Depends on D154366

Depends on D154367

Depends on D154369

Depends on D154370