support nested VM barrier synchronization #889

Job	Run time
omnibus compilations, trigger	2s
omnibus compilations, #2	12m 10s
omnibus compilations, #3	39m 22s
omnibus compilations, #1	33m 51s
omnibus compilations, #4	13m 20s
omnibus compilations, #12	33m 7s
omnibus compilations, #9	36m 17s
omnibus compilations, #6	36m 30s
omnibus compilations, #7	44m 32s
omnibus compilations, #5	1m 53s
omnibus compilations, #8	36m 43s
omnibus compilations, #10	35m 49s
omnibus compilations, #11	10m 6s
omnibus compilations, result	6s
	5h 33m 48s

Provide feedback