Pointer Tagging Techniques in C++: Utilizing Unused Bits for Memory Optimization
By
signa11
Master baker tier. Every paragraph earns its place on the tray.
Summary
This technical article explores pointer tagging in C++ programming, explaining how modern 64-bit systems leave unused bits in pointers that can be repurposed to store metadata. The technique leverages the fact that most systems only use 48 bits for virtual addresses and malloc implementations align allocations to 16-byte boundaries, leaving both upper and lower bits available for tagging. Pointer tagging enables memory savings, faster dynamic dispatch, and more compact data structures by storing type information, flags, or other metadata directly within pointer values.
Key quotes
· 4 pulledA 64-bit pointer can address over 18 exabytes (18 billion gigabytes) of memory, which far exceeds the needs of even the most top-end supercomputers
Most modern desktop CPUs only use 48 bits for virtual addresses, leaving the upper 16 bits unused
Most malloc implementations align allocations to 16-byte boundaries, so the bottom four bits are always set to zero
These bottom four bits can be repurposed to store extra data, as long as those bits are cleared back to zero when dereferencing the pointer
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