Loop Tree Optimization for Control Overhead and Buffer Limits
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Solution Overview
Problem
Existing methods for reducing control overheads, such as loop unrolling and loop splitting, compete for space in instruction buffers limited in size, leading to suboptimal performance in programmable accelerators.
Innovation Solution
Construct a loop tree from a program, compute a metric for each node, and perform loop splitting or unrolling on nodes with the highest benefit ratio of execution cycles gained to code size increase, ensuring the resulting code fits within the buffer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If loop unrolling transformation is performed, then control overheads are reduced, but code size increases and may exceed instruction buffer capacity
Solution Approach 1:
The patent applies segmentation by dividing the loop transformation process into distinct phases: loop splitting first divides the original loop into multiple sub-loops, then loop unrolling is applied selectively to each sub-loop. This segmented approach allows control overhead reduction while managing code size growth through controlled unrolling of individual sub-loops rather than the entire loop structure.
Solution Approach 2:
The patent implements preliminary action by performing loop splitting before loop unrolling. This preliminary transformation restructures the loop into smaller sub-loops with reduced iteration counts, which then makes the subsequent unrolling transformation more manageable and less likely to exceed instruction buffer capacity while still achieving control overhead reduction.
2Productivity
If loop splitting transformation is performed, then control overheads are reduced, but code size increases and may exceed instruction buffer capacity
Solution Approach 1:
The patent applies segmentation by combining loop splitting with loop unrolling in a coordinated transformation process. The loop is first split into sub-loops to reduce control overhead, then unrolling is applied to expand iterations within each sub-loop. This segmented transformation approach manages code size by distributing the transformation across multiple stages rather than applying one large transformation.
Solution Approach 2:
The patent implements preliminary action by performing loop splitting as a preliminary transformation before applying loop unrolling. This preliminary restructuring creates smaller, more manageable sub-loops that when subsequently unrolled produce less code growth than unrolling the original large loop directly, while still achieving the control overhead reduction benefits of both transformations.
3Productivity
If both loop unrolling and loop splitting are performed sequentially, then control overheads are reduced, but instruction buffer capacity is exceeded
Solution Approach 1:
The patent applies segmentation by dividing the transformation process into sequential phases where loop splitting is applied first to create sub-loops, then loop unrolling is applied to each sub-loop individually. This segmented approach to transformation allows the compiler to manage instruction buffer usage by processing smaller units rather than transforming the entire loop structure at once, preventing buffer overflow while achieving cumulative control overhead reduction.
Solution Approach 2:
The patent implements preliminary action by performing loop splitting before loop unrolling. This preliminary transformation reduces the iteration count of each sub-loop before unrolling is applied, which limits the code size growth from unrolling. The preliminary splitting action ensures that subsequent unrolling operates on smaller scopes, keeping total instruction buffer usage within capacity limits while still achieving control overhead reduction.
Data Source
AI summary
Described are techniques for reducing overhead controls. A loop tree is constructed from a program, such as a structured control flow program. Structured control flow refers to a programming concept where the flow of control to a block or region is based on single entry and single-exist methodology (SESE). A loop tree refers to a tree-like data structure that graphically represents loop(s) and/or an if-condition(s) in a program, such as a structured control flow program. A loop splitting operation or a loop unrolling operation may then be performed in connection with the node of the loop tree that is identified as having the highest benefit (ratio of execution cycles gained to the increase in code size) representing an if-condition or a loop, respectively, provided that the resultant code fits in the instruction buffer.