Compute Unit Wavefront Sorting for SIMD Divergence Reduction
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Solution Overview
Problem
Single-instruction multiple-data (SIMD) processors experience a slowdown due to divergent control flow, where different execution paths are serialized, leading to inefficiencies in parallel processing.
Innovation Solution
Reorganize execution items across wavefronts of a workgroup or within a wavefront by identifying control flow targets and sorting execution items based on these targets, reducing divergence by grouping similar paths together.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If execution items are executed in parallel using SIMD architecture, then processing throughput is improved, but control flow divergence causes serialization and reduces efficiency
Solution Approach 1:
The patent applies preliminary action by sorting execution items before they reach the divergent control flow point. The sorting is performed in advance based on predicted control flow targets, so that when divergence occurs, execution items with the same target are already grouped together in contiguous lanes. This pre-organization reduces the need for serialization during actual execution, maintaining parallel throughput while minimizing execution time loss.
Solution Approach 2:
The patent segments execution items into groups based on their control flow targets. By dividing the execution item stream into segments that share common control flow destinations, the system can process each segment in parallel without serialization. This segmentation allows different groups of execution items to follow different control paths simultaneously, resolving the contradiction between parallel throughput and execution time.
2Reliability
If control flow paths are serialized to handle divergence, then correctness is maintained, but processing efficiency deteriorates
Solution Approach 1:
The patent applies dynamics by making the control flow behavior adaptive based on the sorted arrangement of execution items. The system dynamically determines which execution items can proceed in parallel versus which require serialization, based on their grouped control flow targets. This dynamic approach maintains correctness by properly handling divergence while maximizing parallel efficiency by allowing contiguous lanes with the same target to execute simultaneously.
Solution Approach 2:
The patent applies local quality by allowing different control flow behaviors for different groups of execution items. Execution items with the same control flow target maintain full parallelism, while items with different targets are serialized only when necessary. This localized optimization ensures correctness for each control path while preserving parallel efficiency where possible.
Data Source
AI summary
A method and apparatus for reducing divergence of control flow when executing multiple execution items in parallel are disclosed. The method comprises, at a point of divergent control flow, for each execution item, identifying a control flow target that designates a respective post-divergence code path; sorting the execution items in accordance with the identified control flow targets to obtain sorted execution-item groups; redistributing the execution items between distinct wavefronts of a workgroup or different time slots within a wavefront so that, within at least one wavefront or time slot, a greater proportion of the execution items share a common control flow target than prior to the redistribution; and continuing execution of the execution items after the point of divergent control flow using the redistributed execution items.