Branch Prediction With Speculative Indexing For Lower Latency
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Solution Overview
Problem
Existing branch prediction systems face inefficiencies due to static history lengths, larger storage footprints, slower learning, and latency issues when incorporating pattern-based predictors, and lack a method to integrate them into primary predictor caches without extra latency.
Innovation Solution
A branch prediction system that speculatively indexes pattern-based predictors using a dynamic history length, defined by an anchor point, and generates indices based on instruction address, global path vector, and intraline count, allowing simultaneous use of multiple read ports to select predictions immediately or hold them for later, thus integrating pattern-based predictors into primary predictor caches without additional latency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pattern-based predictors are incorporated into primary predictor caches, then branch prediction accuracy is improved, but latency increases
Solution Approach 1:
The patent generates both intraline and new line indices in advance, before the prediction result is needed. This preliminary indexing allows the pattern-based predictor to be ready immediately when a branch instruction is encountered, eliminating the latency that would otherwise result from computing the index at prediction time. The indices are generated speculatively using the current instruction address and global path vector, and the appropriate one is selected based on whether the branch stays within the same line or transitions to a new line.
Solution Approach 2:
The patent implements dynamic history length by using a variable number of global path vector bits in the index generation. Instead of using a fixed history length, the system dynamically adjusts how many recent branch outcomes are incorporated into the index, allowing the predictor to adapt to different code patterns and optimization scenarios. This dynamic approach enables the system to balance between using sufficient history for accuracy while avoiding excessive history that would increase complexity and latency.
2Device complexity
If static history length is used in branch prediction, then device complexity is reduced, but learning speed decreases
Solution Approach 1:
The patent implements dynamic history length by using a variable number of global path vector bits in the index generation. Instead of using a fixed history length, the system dynamically adjusts how many recent branch outcomes are incorporated into the index, allowing the predictor to adapt to different code patterns and optimization scenarios. This dynamic approach enables the system to balance between using sufficient history for accuracy while avoiding excessive history that would increase complexity and latency.
3Device complexity
If pattern-based predictors are integrated without speculative indexing, then device complexity is reduced, but prediction speed decreases
Solution Approach 1:
The patent generates both intraline and new line indices in advance, before the prediction result is needed. This preliminary indexing allows the pattern-based predictor to be ready immediately when a branch instruction is encountered, eliminating the latency that would otherwise result from computing the index at prediction time. The indices are generated speculatively using the current instruction address and global path vector, and the appropriate one is selected based on whether the branch stays within the same line or transitions to a new line.
Data Source
AI summary
A method of branch prediction in a processor includes: generating a new line index and an intraline index; generating a first output of a pattern-based predictor structure using the new line index; generating a second output of the pattern-based predictor structure using the intraline index; selecting one of the first output and the second output based on a result of a prediction pipeline; and predicting a direction of a branch using the selected one of the first output and the second output.