Compare-and-swap - Extensions

Extensions

Since CAS operates on a single pointer-sized memory location, while most lock-free and wait-free algorithms need to modify multiple locations, several extensions have been implemented.

  • Double compare-and-swap compares two unrelated memory locations with two expected values, and if they're equal, sets both locations to new values. This operation only exists on Motorola 680x0 processors, but in early papers was often required.
  • Double-wide compare-and-swap operates on two adjacent pointer-sized locations (or, equivalently, one location twice as big as a pointer). On later x86 processors, the CMPXCHG8B and CMPXCHG16B instructions serve this role, although early 64-bit AMD CPUs did not support CMPXCHG16B (modern AMD CPUs do).
  • Single compare, double swap compares one pointer but writes two. The Itanium's cmp8xchg16 instruction implements this, where the two written pointers are adjacent.
  • Multi-word compare-and-swap is a generalisation of normal compare-and-swap. It can be used to atomically swap an arbitrary number of arbitrarily located memory locations. Usually, multi-word compare-and-swap is implemented in software using normal double-wide compare-and-swap operations. The drawback of this approach is a lack of scalability.

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