[Vm-dev] Why does updating the root table use a red zone?

[bryce at kampjes.demon.co.uk]

I'm not sure why noteAsRoot:headerLoc: uses a red zone. Why can't it
just not update the root table? The root table is only used for
incremental GCs and they will perform a full GC if the root table
has overflowed?

I'm asking because I had a bug in Exupery's version of this which I
fixed by setting allocationCount on root table overflow. This appears
to work but is there a good reason for the logic that the interpreter
uses now?

Bryce

P.S. The method is:
noteAsRoot: oop headerLoc: headerLoc 
	"Record that the given oop in the old object area points to an 
	object in the young area. 
	HeaderLoc is usually = oop, but may be an addr in a 
	forwarding block."
	| header |
	self inline: true.
	header := self longAt: headerLoc.
	(header bitAnd: RootBit) = 0
		ifTrue: ["record oop as root only if not already recorded"
			rootTableCount < RootTableRedZone
				ifTrue: ["record root if there is enough room in the roots 
					table "
					rootTableCount := rootTableCount + 1.
					rootTable at: rootTableCount put: oop.
					self longAt: headerLoc put: (header bitOr: RootBit)]
				ifFalse: ["we're getting in the red zone"
					rootTableCount < RootTableSize
						ifTrue: ["but there's still space to record it"
							rootTableCount := rootTableCount + 1.
							rootTable at: rootTableCount put: oop.
							self longAt: headerLoc put: (header bitOr: RootBit).
							"but force an IGC on the next allocation"
							allocationCount := allocationsBetweenGCs + 1]]]