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When designing concurrent algorithms, Load-Link/Store-Conditional (LL/SC) isoften the ideal primitive to have because unlike Compare and Swap (CAS), LL/SCis immune to the ABA problem. However, the full semantics of LL/SC are notsupported by any modern machine, so there has been a significant amount of workon simulations of LL/SC using Compare and Swap (CAS), a synchronizationprimitive that enjoys widespread hardware support. All of the algorithms so farthat are constant time either use unbounded sequence numbers (and thus baseobjects of unbounded size), or require $\Omega(MP)$ space for $M$ LL/SC object(where $P$ is the number of processes). We present a constant timeimplementation of $M$ LL/SC objects using $\Theta(M+kP^2)$ space, where $k$ isthe maximum number of overlapping LL/SC operations per process (usually aconstant), and requiring only pointer-sized CAS objects. Our implementation canalso be used to implement $L$-word $LL/SC$ objects in $\Theta(L)$ time (forboth $LL$ and $SC$) and $\Theta((M+kP^2)L)$ space. To achieve these bounds, webegin by implementing a new primitive called Single-Writer Copy which takes apointer to a word sized memory location and atomically copies its contents intoanother object. The restriction is that only one process is allowed towrite/copy into the destination object at a time. We believe this primitivewill be very useful in designing other concurrent algorithms as well.

LL/SC and Atomic Copy: Constant Time, Space Efficient Implementations using only pointer-width CAS