Actually, please do not disregard this question, it appears James and I cannot correctly do bitwise arithmetic :)
Brian On Wed, Sep 5, 2012 at 9:34 PM, Brian Geffon <bri...@apache.org> wrote: > Disregard this question, it's not doing what I originally expected it > was doing based on the name. > > Brian > > On Wed, Sep 5, 2012 at 8:45 PM, Brian Geffon <bri...@apache.org> wrote: >> Hi, >> I was curious if anyone could shed some light on the following >> question I've had. So if I understand this correctly the freelists use >> the upper 16bits of the data ptr to be a "version," where this version >> tries to solve several problems related to lockfree data structures >> including the ABA problem and the delete after remove problem. So in >> my quest to understand these more, I'm trying to figure why the macros >> are so complex, for example: >> >> #define FREELIST_POINTER(_x) ((void*)(((((intptr_t)(_x).data)<<16)>>16) | \ >> (((~((((intptr_t)(_x).data)<<16>>63)-1))>>48)<<48))) >> >> Since the pointer is just the lower 48 bits, isn't it enough to just >> do something like: >> >> (void*)(((intptr_t)(_x).data) & (intptr_t)(0x0000FFFFFFFFFFFFULL))) >> >> It would seem like we should be able to pull this off with only a >> single machine instruction? Which is surprising that we dont when the >> freelist version macro is much simpler: >> >> #define FREELIST_VERSION(_x) (((intptr_t)(_x).data)>>48) >> >> I know some pretty smart people worked on this stuff so I'm sure there >> is a very good reason for the complexity, but I just cant figure it >> out. Anyone? >> >> Brian
