Control Flow Handling in Data-Parallel Architectures
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Solution Overview
Problem
Existing methods for handling control flow structures in data-parallel architectures face limitations, such as high overhead in predication for complex structures and limited efficiency due to divergent PC (DPC) units in branching approaches, which affect performance in multithreaded processors like GPUs.
Innovation Solution
A method that combines predication and branching by determining the innermost convergent regions with the highest divergent depth for predication, using free predication to avoid warp register spillage and selectively applying predication to regions based on scalar register pressure, thereby improving performance by optimizing the use of both software-based and hardware-based solutions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If predication is applied to complex control flow structures, then execution speed is improved, but register overhead increases and register spillage occurs
Solution Approach 1:
The patent segments control flow structures into different types (mergeable vs. non-mergeable) and applies different handling strategies to each. Mergeable control flow structures are handled through predication with lane mask merging, while non-mergeable structures use traditional branching. This segmentation allows predication to be applied selectively to reduce overhead rather than universally, thus improving speed without excessive register overhead.
Solution Approach 2:
The patent applies predication partially - only to mergeable control flow structures where it is beneficial, rather than applying it excessively to all control flow structures. By identifying and selectively predating only those structures that can merge lane masks efficiently, the system achieves speed improvement without incurring register spillage from over-predication.
2Ease of manufacture
If branching is used to handle control flow structures, then implementation simplicity is maintained, but performance is limited due to divergent PC units
Solution Approach 1:
The patent combines predication and branching into a hybrid approach. Mergeable control flow structures use predication to improve performance by eliminating DPC unit requirements, while non-mergeable structures continue to use branching for implementation simplicity. This merging of two approaches allows the system to achieve high performance where possible while maintaining simplicity where predication would be problematic.
Solution Approach 2:
The patent applies different quality characteristics to different parts of the control flow handling system. For mergeable control flow structures, it applies the high-performance predication mechanism. For non-mergeable structures, it maintains the simple branching mechanism. This local differentiation optimizes performance for suitable cases while preserving implementation simplicity for cases where predication would be complex.
3Productivity
If predication is applied to all control flow structures, then execution efficiency is improved, but warp register pressure increases causing register spillage
Solution Approach 1:
The patent segments control flow structures into mergeable and non-mergeable categories, applying predication only to the mergeable segment. This segmentation prevents the excessive accumulation of warp register pressure that would occur if predication were applied universally, while still achieving execution efficiency improvement in the appropriate segments.
Solution Approach 2:
The patent applies predication partially - only to the extent necessary and beneficial for mergeable control flow structures. By avoiding predication on non-mergeable structures, the system prevents excessive warp register pressure accumulation and associated register spillage, while maintaining execution efficiency where predication is effective.
Data Source
AI summary
Methods, systems and apparatus are provided for handling control flow structures in data-parallel architectures. A method includes receiving, by a processing unit (PU), a program for execution. The method further includes applying, by the PU, a branching solution to the program to obtain data on control flow structures of the program. The method further includes determining, by the PU and based at least in part on the obtained data, one or more control flow structures of the program to predicate. The method further includes applying, by the PU, predication to the one or more control flow structures of the program.