Fix inaccurate Always_Terminates aspect on atomic operations - #30
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Atomic exchange intrinsics are used in package LibSAP.Pointer_Holders, and Always_Terminates was specified for these operations. This is inaccurate, however, since atomic exchange is not guaranteed to terminate on all target architectures. For example, on Armv7-M, the compiler emits LDREX and STREX instructions in a loop to implement atomic exchange. Such a loop is not guaranteed to terminate as it may experience a livelock or infinite retry loops. In practice this should not happen, but the worst case must be specified for the termination proofs to be correct.
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Atomic exchange intrinsics are used in package LibSAP.Pointer_Holders, and Always_Terminates was specified for these operations. This is inaccurate, however, since atomic exchange is not guaranteed to terminate on all target architectures.
For example, on Armv7-M, the compiler emits LDREX and STREX instructions in a loop to implement atomic exchange. Such a loop is not guaranteed to terminate as it may experience a livelock or infinite retry loops. In practice this should not happen, but the worst case must be specified for the termination proofs to be correct.