1. A computer-implemented method, comprising: executing, in an execution environment, one or more transactions in a first transactional memory mode, wherein while in said first transactional memory mode all transactions executed in the execution environment are executed according to a particular transactional memory implementation; dynamically transitioning to a second transactional memory mode during runtime; and executing one or more transactions in the second transactional memory mode, wherein while in said second transactional memory mode all transactions executed in the execution environment are executed according to a different transactional memory implementation than in the first transactional memory mode; wherein said particular transactional memory implementation and said different transactional memory implementation respectively provide for atomic execution of respective sequences of instructions with respect to a shared transactional memory space.