Programming a Multicore Architecture without Coherency and Atomic Operations
Proceedings of Programming Models and Applications on Multicores and Manycores
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Porting software to different platforms can require modifications of the application. One of the issues is that the targeted hardware supports another memory consistency model. As a consequence, the completion order of reads and writes in a multi-threaded application can change, which may result in improper synchronization. For example, a processor with out-of-order execution could break synchronization if proper fence instructions are missing. Such a bug can cause sporadic errors, which are hard to debug. This paper presents an approach that makes applications independent of the memory model of the hardware, hence they can be compiled to hardware with any memory architecture. The key is having a memory model that only guarantees the most fundamental orderings of reads and writes, and annotations to specify additional ordering constraints. As a result, tooling can transparently and properly implement fences, cache flushes, etc. when appropriate, without losing flexibility of the hardware design. In a case study, several SPLASH-2 applications are run on a 32-core software cache coherent Micro Blaze system in FPGA. Moreover, this approach also allows mapping to scratch-pad memories and a distributed shared memory architecture.