Modulo scheduling with reduced register pressure

Software pipelining is a scheduling technique that is used by some product compilers in order to expose more instruction level parallelism out of innermost loops. Module scheduling refers to a class of algorithms for software pipelining. Most previous research on module scheduling has focused on reducing the number of cycles between the initiation of consecutive iterations (which is termed II) but has not considered the effect of the register pressure of the produced schedules. The register pressure increases as the instruction level parallelism increases. When the register requirements of a schedule are higher than the available number of registers, the loop must be rescheduled perhaps with a higher II. Therefore, the register pressure has an important impact on the performance of a schedule. This paper presents a novel heuristic module scheduling strategy that tries to generate schedules with the lowest II, and, from all the possible schedules with such II, it tries to select that with the lowest register requirements. The proposed method has been implemented in an experimental compiler and has been tested for the Perfect Club benchmarks. The results show that the proposed method achieves an optimal II for at least 97.5 percent of the loops and its compilation time is comparable to a conventional top-down approach, whereas the register requirements are lower. In addition, the proposed method is compared with some other existing methods. The results indicate that the proposed method performs better than other heuristic methods and almost as well as linear programming methods, which obtain optimal solutions but are impractical for product compilers because their computing cost grows exponentially with the number of operations in the loop body.Peer ReviewedPostprint (published version

Fine grain software pipelining of non-vectorizable nested loops

This paper presents a new technique to parallelize nested loops at the statement level. It transforms sequential nested loops, either vectorizable or not, into parallel ones. Previously, the wavefront method was used to parallelize non-vectorizable nested loops. However, in order to reduce the complexity of parallelization, the wavefront method regards an iteration as an unbreakable scheduling unit and draws parallelism through iteration overlapping. Our technique takes a statement rather than an iteration as the scheduling unit and exploits parallelism by overlapping the statements in all dimensions. In this paper, we show how this finer grain parallelization can be achieved with reasonable computational complexity, and the effectiveness of the resulting method in exploiting parallelism

Software Pipelining in the LLVM Compiler

Tahle práce pojednává o návrhu a implementaci techniky programového zřetězení aneb Software pipelining, optimalizaci cyklů v programu, která se snaží plně využít paralelismus na úrovni instrukcí. To dosahuje plánovaním instrukcí způsobem, aby se jednotlivé iterace cyklu překrývaly a bylo je možné vykonávat zřetězeně. Optimalizace takhle zvyšuje rychlost výsledného programu. Je tu popsaný návrh a implementace algoritmu Swing Modulo Scheduling, efektivní metody pro nacházení optimálního plánu pro zřetězení cyklů. Práce byla vytvořena jako součást většího projektu a to vývoje Codasip Framework. Jeho součástí je překladač jazyka C do jazyka symbolických instrukcí vytvořený nad překladačovou architekturou LLVM. V tomto překladači je implementován výsledek této práce.This thesis discusses a design and implementation of the Software Pipelining, a optimization technique of loops in a program, which tries to exploit instruction-level parallelism. It is achieved by scheduling instructions in a way to overlap iterations of the loop and therefore execute them in a pipeline. This way optimization speeds up the final program. There is a detailed description of design and implementation of Swing Modulo Scheduling algorithm, an effective and efficient method for finding near-optimal plans for software-pipelined loops. This work has been done as a part of a larger project, the development of Codasip Framework. Part of this framework is the retargetable C compiler based on compiler architecture LLVM, in which this work is implemented.