ITSLF: Inter-Thread Store-to-Load Forwardingin Simultaneous Multithreading

In this paper, we argue that, for a class of fine-grain, synchronization-intensive, parallel workloads, it is advantageous to consolidate synchronization and communication as much as possible among the threads of simultaneous multithreading (SMT) cores. While, today, the shared L1 is the closest coherent level where synchronization and communication between SMT threads can take place, we observe that there is an even closer shared level, entirely inside a single core. This level comprises the load queues (LQ) and store queues (SQ) / store buffers (SB) of the SMT threads and to the best of our knowledge it has never been used as such. The reason is that if we allow communication of different SMT threads via their LQs and SQs/SBs, i.e., inter-thread store-to-load forwarding (ITSLF), we violate write atomicity with respect to the outside world, beyond the acceptable model of read-own-write-early multiple-copy atomicity (rMCA).