1. A method, comprising: performing, by a computer: executing, in an execution environment, a plurality of 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 in response to said dynamically transitioning: executing a plurality of 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.