Multi-Level PHT Swapping for Low-Latency Branch Prediction
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Solution Overview
Problem
Existing branch prediction systems face inefficiencies in managing data swaps between first and second-level pattern history tables (PHT-1 and PHT-2) due to latency issues and inaccurate predictions, leading to performance bottlenecks in core execution pipelines.
Innovation Solution
A branch prediction logic system utilizing a multi-level PHT structure with a line presence bit (LPB) in a prediction update queue (PUQ) to determine data swaps between PHT-1 and PHT-2 based on the presence of metadata, optimizing data eviction and reducing latency penalties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If data swaps between PHT-1 and PHT-2 are performed frequently to maintain prediction accuracy, then branch prediction accuracy is improved, but latency increases due to swap operations
Solution Approach 1:
The patent implements dynamic data swap operations between PHT-1 and PHT-2 based on line presence bit (LPB) status and metadata availability. The system adaptively determines when to swap data, transitioning from static to dynamic swap timing. This resolves the contradiction by making the swap frequency conditional rather than fixed, improving accuracy only when necessary while minimizing latency penalties.
Solution Approach 2:
The system changes the parameter of data swap timing based on the LPB state and metadata presence. When LPB indicates a hit in PHT-2, the system triggers a swap to bring updated data to PHT-1. This parameter-based control mechanism optimizes the trade-off between maintaining prediction accuracy and minimizing swap-induced latency.
2Measurement precision
If a multi-level PHT structure is implemented to improve prediction accuracy, then branch prediction accuracy is improved, but device complexity increases
Solution Approach 1:
The patent divides the PHT into two levels: PHT-1 for frequently accessed branch data and PHT-2 for less frequently accessed or backup data. This segmentation allows the system to maintain high prediction accuracy by having multiple specialized tables while managing complexity through clear division of functions. Each level serves a specific purpose, reducing the burden on any single table.
Solution Approach 2:
The multi-level PHT structure implements a nested hierarchy where PHT-2 serves as a backup or extension to PHT-1. Data can be swapped between levels, creating a nested relationship similar to the doll principle. This nesting allows the system to achieve higher accuracy through multiple layers of prediction data while managing complexity through hierarchical organization rather than parallel independent structures.
3Loss of time
If data is retained in PHT-2 to avoid swaps, then latency is reduced, but prediction accuracy deteriorates due to outdated data
Solution Approach 1:
The system performs preliminary actions by pre-loading or pre-swapping data between PHT-1 and PHT-2 based on predicted branch patterns. The line presence bit mechanism anticipates when data will be needed and prepares swaps in advance, reducing the impact of swap operations on latency while ensuring data freshness for accurate predictions.
Solution Approach 2:
The patent implements a feedback mechanism through the line presence bit that monitors the state of PHT-2 and triggers swaps when outdated data is detected. This feedback loop ensures that PHT-1 contains current data for accurate predictions while PHT-2 serves as a buffer, dynamically adjusting the balance between latency and accuracy based on actual data freshness requirements.
Data Source
AI summary
A branch prediction logic system includes a branch history table (BHT), a multi-level history table, and a prediction update queue. The BHT includes a plurality of lines, each line corresponding to at least one branch instruction and containing history information specific to the at least one branch instruction. A first pattern history table (PHT-1) stores first branch data corresponding to the at least one branch instruction included in a given line of the BHT and a second pattern history table (PHT-2) stores second branch data corresponding to the at least one branch instruction included in a given line. The prediction update queue stores a line presence bit having one of a “1” logic state or a “0” logic state. The branch prediction logic system performs a data swap between the PHT-2 and the PHT-1 based on the logic state of the line presence bit.