Using hardware-transactional-memory support to implement speculative task execution

Loops take up most of the time of computer programs, so optimizing them so that they run in the shortest time possible is a continuous task. However, this task is not negligible; on the contrary, it is an open area of research since many irregular loops are hard to parallelize. Generally, these loops have loop-carried (DOACROSS) dependencies and the appearance of dependencies could depend on the context. Many techniques have been studied to be able to parallelize these loops efficiently; however, for example in the OpenMP standard there is no efficient way to parallelize them. This article presents Speculative Task Execution (STE), a technique that enables the execution of OpenMP tasks in a speculative way to accelerate certain hot -code regions (such as loops) marked by OpenMP directives. It also presents a detailed analysis of the application of Hardware Transactional Memory (HTM) support for executing tasks speculatively and describes a careful evaluation of the implementation of STE using HTM on modern machines. In particular, we consider the scenario in which speculative tasks are generated by the OpenMP taskloop construct ( Speculative Taskloop (STL) ). As a result, it provides evidence to support several important claims about the performance of STE over HTM in modern processor architectures. Experimental results reveal that: (a) by implementing STL on top of HTM for hot -code regions, speed-ups of up to 5.39x can be obtained in IBM POWER8 and of up to 2.41x in Intel processors using 4 cores; and (b) STL-ROT, a variant of STL using rollback-only transactions (ROTs), achieves speed-ups of up to 17 .70x in IBM POWER9 processor using 20 cores.