Comment by jcranmer
8 hours ago
CHERI has 64-bit object size but 128-bit pointers (because the pointer values also carry pointer provenance metadata in addition to an address). I know some of the pointer types on GPUs (e.g., texture pointers) also have wildly different sizes for the address size versus the pointer size. Far pointers on segmented i386 would be 16-bit object and index size but 32-bit address and pointer size.
There was one accelerator architecture we were working that discussed making the entire datapath be 32-bit (taking less space) and having a 32-bit index type with a 64-bit pointer size, but this was eventually rejected as too hard to get working.
I guess today, instead of 128bit pointers we have 64bit pointers and secret provenance data inside the cpu, at least on the most recent shipped iPhones and Macs.
In the end, I’m not sure that’s better, or maybe we should have had extra large pointers again (in that way back 32bit was so large we stuffed other stuff in there) like CHERI proposes (though I think it still has secret sidecar of data about the pointers).
Would love to Apple get closer to Cheri. They could make a big change as they are vertically integrated, though I think their Apple Silicon for Mac moment would have been the time.
I wonder what big pointers does to performance.
It's not secret, it just reuses some of the unused address bits.