Incorporating speculative execution in exact control-dependent scheduling

Abstract

-This paper describes a symbolic formulation that allows incorporation of speculative operation execution (preexecution) in an exact control-dependent scheduling of arbitrary forward branching control/data paths. The technique provides a closed form solution set in which all satisfying schedules are encapsulated in a compressed OBDD-based representation. To extract parallelism implicit in the input specification Boolean 'guard' functions are used to identify paths where operations have to be scheduled and the execution order of the conditionals is dynamically resolved. An efficient and systematic iterative construction method is presented along with benchmark results.

Hierarchical conditional dependency graphs as a unifying design representation in the CODESIS high-level synthesis system

“…In our previous work of [22], [19] we aligned with the view supported by others [5], [8], [9], in advocating for the need of more flexible internal design representations, to optimize the HLS results and effectively handle both control and data flow dominated designs.…”

“…Guards (named H i ) are a special type of nodes (shown as rectangles) and represent boolean conditions that control the execution of operations and the assignment of values to variables. They have also been used as a formal control model in [4], [5] using a CDFG representation.…”

“…Control representations based on BDD's have already been used in previous work ( [15], [4], [5]). The originality of the GH lies on the hierarchy construction and not at the use of BDD's which are simply used for their efficiency.…”

“…In respect to [9] and [5], speculative execution is considered only after normally executing nodes have been scheduled. In this way the risk of lengthening execution paths by displacing normally executing operations in favor of speculatively executing ones, is avoided.…”

“…Several better adapted heuristics were proposed ( [1], [2], [3], [4], [5], [6]). The quality of their results depends heavily on the ability to exploit conditional resource sharing ( [2], [4], [6], [7]) and speculative execution ( [3], [5], [16], [17]) possibilities as well as shorten path lengths using node duplication techniques [3].…”