> Would appreciate hearing from an expert about why this is a thing. Also, if the addresses are virtualized, why not take the memory savings that'd come with ef

> Would appreciate hearing from an expert about why this is a thing. Also, if the addresses are virtualized, why not take the memory savings that'd come with efficiently packing the addresses into only 48-bytes? Not an expert at all, but the the place where I've really used this sort of method is when writing a GC and trying to figure out where to store a "pointer has been visited" marker in the pointer instead of in a different data structure. This was also on ARM, but quite a long time ago on a Sharp Zaurus. The word aligned pointer model gave 2 bits at the end of the pointer to mark cycles in GC pathway. This was much simpler than a separate data structure to keep the mark phase of the traversal in memory & was an extension of the "1 bit reference counter" paper[1] with white, gray and black for the bits in there. Of course, you need to go back and rewrite them out to 00, but with virtual memory in the mix you can play bigger tricks. This does save a lot of memory, hash lookups and is very low-overhead way of keeping a 1:1 metadata storage for every pointer you have. You'll see the same trick in the JVM ZGC where higher bits are used to keep track of the metadata about the items being collected [2]. With a little bit of pointer arithmetic, you can also turn a pointer read-only to prevent some process from modifying shared data outside of a lock too, to avoid copying data before unlocking. I had a double mmap mode in PHP APC which segv'd when the core engine mutated a cached data structure outside of a lock[3]. [1] - https://link.springer.com/article/10.1007/BF01932156 https://link.springer.com/article/10.1007/BF01932156 [2] - http://hg.openjdk.java.net/zgc/zgc/file/59c07aef65ac/src/hotspot/os_cpu/linux_x86/zGlobals_linux_x86.hpp#l59 http://hg.openjdk.java.net/zgc/zgc/file/59c07aef65ac/src/hot... [3] - https://notmysock.org/blog/2009/Jan/08/ https://notmysock.org/blog/2009/Jan/08/