Fetch Block Branch Prediction for Low-Hardware Pipeline Scaling
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Solution Overview
Problem
Existing pipelined processors with multi-instruction branch prediction logic units are costly, inefficient, and lack scalability due to significant hardware usage.
Innovation Solution
Implement a fetch block-based branch prediction method that generates a predicted target address in one operating clock cycle, using a branch direction predictor and target address predictor to optimize branch direction and target address prediction, reducing hardware requirements and improving efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multi-instruction branch prediction logic units are implemented in pipelined processors, then branch prediction accuracy and parallelism are improved, but hardware usage and manufacturing cost increase significantly
Solution Approach 1:
The patent segments the branch prediction function into two independent units: a branch direction predictor that determines whether a branch is taken or not taken, and a branch target address predictor that predicts the target address. This segmentation allows each unit to be simpler and more specialized, reducing overall hardware complexity while maintaining prediction accuracy through functional division.
Solution Approach 2:
The patent implements a universal branch prediction mechanism that handles multiple branch types (conditional branches, unconditional branches, indirect branches) through a single integrated system. The predictor uses a unified table structure and prediction logic that can accommodate different branch scenarios, eliminating the need for separate prediction logic for each branch type and reducing hardware requirements.
2Measurement precision
If traditional branch prediction methods are used, then branch direction can be predicted, but target address prediction is inaccurate and requires additional hardware
Solution Approach 1:
The patent merges the branch direction prediction and target address prediction functions into a single integrated predictor structure. The branch direction predictor and branch target address predictor share common resources such as the prediction table and index generation logic, allowing accurate target address prediction without requiring separate dedicated hardware for each function.
Solution Approach 2:
The patent uses a simplified predictor table structure that stores prediction information in a compact format. Instead of storing complete target address information for all possible branches, the system uses a copy-based approach where prediction entries are replicated and reused for similar branch patterns, reducing the amount of hardware storage required while maintaining prediction accuracy.
3Productivity
If branch prediction logic is replicated for multi-instruction pipelining, then instruction pipeline parallelism is improved, but manufacturing cost and scalability are worsened
Solution Approach 1:
The patent changes the parameters of the branch prediction system by using a table-based prediction mechanism with configurable table size and entry format. This parameter-based approach allows the same basic predictor structure to be adapted for different pipeline depths and instruction widths by simply adjusting table dimensions and indexing parameters, making the system scalable and easier to manufacture for different processor configurations.
Data Source
AI summary
A computer-implemented method of predicting a branch direction of a fetch block in a processor, includes in part, determining a multitude of first counts each associated with a different one of a multitude of branch offsets of a branch direction predictor data associated with the fetch block. Each of the multitude of first counts represents the number of times that the associated branch offset was taken during a multitude of fetch cycles. The computer-implemented method further includes, in part, determining a second count associated with the fetch block. The second count represents the number of times that none of the multitude of branch offsets were taken during the multitude of fetch cycles. The computer-implemented method further includes, in part, computing a confidence level based on the multitude of first counts and the second count, and determining the branch direction of the fetch block in accordance with the computed confidence level.