VPB_SHARERS_SHIFT
(((x) & ~VPB_BIT_FLAGMASK) >> VPB_SHARERS_SHIFT)
#define VPB_SHARERS_WORD(x) ((x) << VPB_SHARERS_SHIFT | VPB_BIT_SHARED)
#define VPB_ONE_SHARER (1 << VPB_SHARERS_SHIFT)