Trace Cache Filter for Internal Branch Inclusion
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Solution Overview
Problem
Traditional processor designs face challenges in implementing trace caches due to the cost of handling unexpected branch exits and the substantial processor area and power required by trace cache and its control circuitry.
Innovation Solution
The proposed solution involves configuring control circuitry for a trace cache to assemble traces with restricted internal conditional branches, designating stable branches based on execution history, and using a biased branch predictor to improve trace cache performance and reduce complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If trace cache is implemented with traditional designs, then instruction fetch bandwidth is improved, but processor area and power consumption increase substantially
Solution Approach 1:
The patent segments the trace cache into multiple sets (e.g., 4 sets) with a smaller number of ways each, replacing a traditional single large cache. This segmentation reduces the total processor area required while maintaining the instruction fetch bandwidth improvement benefits of trace caching.
Solution Approach 2:
The patent uses a biased branch predictor that only predicts certain branches (those with historical bias) rather than all branches. This partial action approach reduces the complexity and area of the branch predictor circuitry while still capturing the majority of branch behavior, thereby reducing overall processor area.
2Productivity
If trace cache is implemented with traditional designs, then instruction fetch bandwidth is improved, but power consumption increases substantially
Solution Approach 1:
By segmenting the trace cache into multiple smaller sets, the patent reduces the total power consumption compared to a single large cache. Each set can be independently managed and powered, allowing for more efficient power distribution and reduced overall energy usage while maintaining fetch bandwidth improvements.
Solution Approach 2:
The biased branch predictor only actively predicts branches with historical bias patterns, reducing the computational overhead and power consumption compared to predicting all branches. This partial prediction approach maintains effective instruction fetch while reducing power consumption.
3Productivity
If trace cache is implemented with traditional designs, then performance is improved, but complexity on the critical path increases
Solution Approach 1:
The biased branch predictor only processes and predicts branches that exhibit historical bias patterns, rather than analyzing all branches. This partial action significantly reduces the complexity on the critical path while maintaining performance benefits for the majority of branch instructions.
Solution Approach 2:
The multi-set trace cache structure simplifies the critical path by allowing independent set management and reducing the complexity of trace selection and replacement logic compared to a single large cache, thereby maintaining performance while reducing critical path complexity.
Data Source
AI summary
Disclosed techniques relate to trace caches. Trace cache circuitry may identify traces that satisfy one or more criteria. Generally, internal branches of a trace should satisfy a threshold bias level in a particular direction. To achieve this goal, the processor may initially assume that branches meet the threshold, track their usefulness in the trace context over time, and prevent inclusion of branches that fall below a usefulness threshold (which indicates that those branches are not sufficiently biased). Branches that do not meet the threshold may be added to a Bloom filter, for example. Usefulness may be tracked during trace training, when valid in a trace cache, or both.