On 5-Dec-05, at 8:45 AM, Bert Freudenberg wrote:

> Just had the VM crash on me - did nothing serious, just debugging  
> some stuff. Any ideas?
Despite the work I did some months ago on the become prim there are  
still pathological cases where I wasn't able to think of a way to  
fully guard the prim.

 From prepareForwardingTableForBecoming: array1 with: array2 twoWay:  
twoWayFlag


	"Important note on multiple references to same object  - since the  
preparation of
	fwdBlocks is NOT idempotent we get VM crashes if the same object is  
referenced more
	than once in such a way as to require multiple fwdBlocks.
	oop1 forwardBecome: oop1 is ok since only a single fwdBlock is needed.
	oop1 become: oop1 would fail because the second fwdBlock woudl not  
have the actual object
	header but rather the mutated ref to the first fwdBlock.
	Further problems can arise with an array1 or array2 that refer  
multiply to the same
	object. This would notbe expected input for programmer writen code  
but might arise from
	automatic usage such as in ImageSegment loading.
	To avoid the simple and rather common case of oop1 become*: oop1, we  
skip such pairs
	and simply avoid making fwdBlocks - it is redundant anyway"

The prim needs some guarding in the image to try to check for problem  
cases. It's just too convoluted to be sensibly done in the VM.

tim
-- 
tim Rowledge; tim at rowledge.org; http://www.rowledge.org/tim