Nested loops represent a significant portion of application runtime in multimedia and DSP applications, an important domain of applications for coarse-grained reconfigurable architectures (CGRAs). While conventional approaches to mapping nested loops utilize only a single-dimensional pipelining, which is either along the innermost loop or along an outer loop, in this paper, we explore an orthogonal approach of pipelining along multiple loop dimensions by first flattening the loop nest. To remedy the inevitable problem of repetitive outer-loop computation in flattened loops, we present a small set of special operations that can effectively reduce the number and frequency of micro-operations in the pipelined loop. We also present a loop transformation technique that can make our special operations applicable to a broader range of loops, including those with triangular iteration spaces. Our experimental results using imperfect loops from StreamIt benchmarks demonstrate that our special operations can cover a large portion of operations in flattened loops, improve performance of nested loops by nearly 30% over using loop flattening only, and achieve near-ideal executions on CGRAs for imperfect loops.

While programmable accelerators such as application-specific processors and reconfigurable architectures can dramatically speed up compute-intensive kernels of an application, application performance can still be severely limited by the communication between processors. To minimize the communication overhead, a shared memory such as a scratchpad memory may be employed between the main processor and the accelerator coprocessor. However, this setup poses a significant challenge to the main processor, which now must manage data on the scratchpad explicitly, resulting in superfluous data copying due to the inflexibility of a scratchpad. In this article, we present an enhancement of a scratchpad, Configurable Range Memory (CRM), whose address range can be reprogrammed to minimize unnecessary data copying between processors and therefore promote data reuse on the accelerator, and also present a software management algorithm for the CRM. Our experimental results involving detailed simulation of full multimedia applications demonstrate that our CRM architecture can reduce the communication overhead quite effectively, reducing the kernel execution time by up to 28% and the application runtime by up to 12.8%, in addition to considerable system energy reduction, compared to the conventional architecture based on a scratchpad.

Control divergence poses many problems in parallelizing loops. While predicated execution is commonly used to convert control dependence into data dependence, it often incurs high overhead because it allocates resources equally for both branches of a conditional statement regardless of their execution frequencies. For those loops with unbalanced conditionals, we propose a software transformation that divides a loop into two or three smaller loops so that the condition is evaluated only in the first loop, while the less frequent branch is executed in the second loop in a way that is much more efficient than in the original loop. To reduce the overhead of extra data transfer caused by the loop fission, we also present a hardware extension for a class of Coarse-Grained Reconfigurable Architectures (CGRAs). Our experiments using MiBench and computer vision benchmarks on a CGRA demonstrate that our techniques can improve the performance of loops over predicated execution by up to 65% (37.5%, on average), when the hardware extension is enabled. Without any hardware modification, our software-only version can improve performance by up to 64% (33%, on average), while simultaneously reducing the energy consumption of the entire CGRA including configuration and data memory by 22%, on average.