Dynamic conditional branch balancing during the high-level synthesis of control-intensive designs

Abstract

We present two novel strategies to increase the scope for application of speculative code motions: (1) Adding scheduling steps dynamically during scheduling to conditional branches with fewer scheduling steps. This increases the opportunities to apply code motions such as conditional speculation that duplicate operations into the branches of a conditional block. (2) Determining if an operation can be conditionally speculated into multiple basic blocks either by using existing idle resources or by creating new scheduling steps. These strategies lead to balancing of the number of steps in the conditional branches without increasing the longest path through the conditional block. Algorithms for these strategies have been implemented within the Spark high-level synthesis framework that accepts a behavioral description in ANSI-C as input and produces synthesizable register-transfer level VHDL. Experiments on two moderately complex industrial-strength applications, namely, MPEG-1 and the GIMP image processing tool, demonstrate that conditional speculation is ineffective without using these strategies.

Formal verification of code motion techniques using data-flow-driven equivalence checking

“…(ii) Duplicating up involves moving operations from a BB in which conditional branches merge to its preceding BBs in the conditional branches. Conditional speculation [Gupta et al 2004] and branch balancing [Gupta et al 2003a] fall under this category. (iii) Boosting up moves operations from a BB within a conditional branch to the BB preceding the CB from which the conditional branch sprouts.…”

Coordinated parallelizing compiler optimizations and high-level synthesis

“…For the other trails, the heuristic calls a procedure that finds a scheduling step (trailStep) in the lastBBInTrail with an un-utilized resource on which operation op can execute. This procedure, FindResSlot, may also insert a new scheduling step into a basic block if no scheduling step is found with an empty resource slot [59]. A duplicate copy of op is then inserted into this trailStep in lastBBInTrail.…”

“…If there is no idle resource, the code motion heuristic employed later by the scheduler may even instruct this function to create new scheduling steps [59]. If the FindResSlot function is unable to accommodate a duplicated copy of op in any of the basic blocks in bbList, then the operation op is removed from the available operation list.…”

“…In this case, the algorithm should traverse the HTG and get the next basic block in the HTG to schedule. However, it is at this point that we employ a novel technique that inserts new scheduling steps in conditional branches that have fewer scheduling steps [59].…”

Dynamically increasing the scope of code motions during the high-level synthesis of digital circuits

“…In this paper, we present algorithms for two techniques that insert new scheduling steps dynamically during scheduling in the shorter of the two branches of a conditional block without increasing the longest path through the conditional [4]. The new scheduling steps, together with idle resources in the basic block of the other conditional branch, can be used to schedule operations by conditional speculation.…”