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Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

tel-00553143, version 1

tel-00553143, version 1 - 6 Jan 2011 58 High-level synthesis and application specific processor design operations are scheduled. 3.1.2.2 ALAP scheduling 1 3 4 1 2 3 2 6 8 10 5 (a) 4 7 (b) 5 7 9 11 6 8 10 9 11 Figure 3.2: (a) ASAP scheduling (b) ALAP scheduling As Late As Possible (ALAP) scheduling has a very similar approach to ASAP scheduling. In contrast to ASAP, ALAP scheduling schedules the operations from the last control step toward the first performing a backward covering. An operation is scheduled to the next control step if all of its successors are scheduled. Figure 3.2 shows an example of ASAP and ALAP scheduling. 3.1.3 Resource-constrained scheduling Scheduling, under resource constraints (whether computation resources or computation time) is a computationally difficult problem that lies in the domain of NP-Complete problems [15]. Some of the mostly used scheduling algorithms and their implementations are discussed in the following paragraphs. 3.1.3.1 List scheduling: One commonly used approach is called“list scheduling”or“resource-constrained scheduling” in which we specify a hardware constraint and use an algorithm to minimize the total execution time that satisfies the given constraint. Like ASAP, the operations in the DFG are assigned to control steps from the first control step to the last. The ready operations are given a priority according to heuristic rules and are scheduled into the

tel-00553143, version 1 - 6 Jan 2011 High-Level Synthesis (HLS) 59 1 2 3 4 2 2 1 1 Figure 3.3: List scheduling with deferred operations. next control step according to this predefined priority. When the number of scheduled operations exceeds the number of resources, the remaining operations are deferred to next control step. Which operations to defer often depends on some local priority such as urgency. In Figure 3.3, for example, two add operations may be scheduled in the first control step, so we must defer one of them. Since they are both on the critical path, they have the same urgency, so we could choose either one. In the figure, the left addition is deferred to the second control step. 3.1.3.2 Force-Directed Scheduling (FDS): The force-directed scheduling (FDS), also called “time-constrained scheduling”, reduces the number of functional units, registers, and busses required. The strategy is to place similar operations in different control steps so as to balance the concurrency of the operations assigned to the units without increasing the total execution time. In FDS, force values are calculated for all operations at all feasible control steps. The pairing of operation and control step that has the most attractive force is selected and assigned. After the assignment, the forces of the unscheduled operations are reevaluated. Assignment and evaluation are iterated until all the operations are assigned. 3.1.3.3 Force-Directed List Scheduling (FDLS): Paulin et el. [106] suggested a hybrid of both of the above-mentioned approaches that is called “force-directed list scheduling (FDLS)”. It provides a solution considering both timing and hardware resource constraints. Similar to list scheduling, FDLS also schedules the operations in the DFG to different control steps from the first control step to the last. However, the local priorities of operations that are used to defer them to next control step are defined by the force function used in FDS, instead of the usual metrics such as urgency. The complexity of both FDS and FDLS is O(n 2 ) where n is number of operations present in a CDFG. However, all the above mentioned algorithms i.e. list scheduling, FDS and FDLS are heuristic algorithms that provide an approximate solution while the optimality of the solution cannot be guaranteed.

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