Some implementations of heap memory allocators manage free memory chunks as doubly-linked lists. There are multiple lists for chunks of equal size, i.e. 4, 8, 1

Some implementations of heap memory allocators manage free memory chunks as doubly-linked lists. There are multiple lists for chunks of equal size, i.e. 4, 8, 16, 32 ... 2^n. This XOR linked list hack makes sense for smallest chunk size. On 32-bit system, two pointers will take 8 bytes, so minimum size of allocated memory block is 8 bytes without this hack and 4 bytes with this hack. On 64-bit system, this hack makes possible allocation of 8-byte block from heap without rounding up to 16 bytes. For other practical purposes, I think that trading speed and code maintainability for memory size isn't good idea.