Processor Branch Target Buffer for Double Prediction Eligibility
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Solution Overview
Problem
Current computer architectures face challenges in improving instruction execution performance due to the uncertainty of branch outcomes before execution, leading to inefficiencies in fetch logic and increased cycle requirements for branch prediction.
Innovation Solution
The implementation of a processor architecture with multiple decode clusters and a branch target buffer that distinguishes between real and fake branch instructions, allowing for double prediction eligibility states to facilitate load balancing and efficient instruction fetching by communicating multiple predictions in a single fetch cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If branch prediction logic is implemented to predict branch outcomes, then instruction fetching efficiency is improved, but device complexity increases
Solution Approach 1:
The patent segments the branch prediction logic into multiple independent prediction queues, each handling specific branch types or prediction scenarios. This segmentation allows the complex prediction function to be divided into manageable units that can operate in parallel, improving instruction fetching efficiency while keeping each individual queue relatively simple in structure.
Solution Approach 2:
The patent introduces a new dimension to branch prediction by implementing multiple prediction queues that operate simultaneously at different levels of prediction granularity. Instead of a single sequential prediction mechanism, the system uses multiple parallel prediction paths, effectively adding a dimensional aspect to the prediction process that improves throughput without proportionally increasing complexity.
2Loss of time
If multiple predictions are communicated in a single fetch cycle, then latency is reduced, but device complexity increases
Solution Approach 1:
The patent merges multiple prediction results into a single fetch cycle by using multiple prediction queues that can be accessed simultaneously. Instead of sequential prediction and fetching, the system combines multiple predictions and merges them into one unified fetch operation, reducing latency while managing complexity through structured access patterns.
Solution Approach 2:
The patent implements preliminary actions by pre-loading multiple predictions into the fetch logic before the actual fetch cycle. The multiple prediction queues prepare and stage prediction results in advance, allowing them to be communicated and applied in a single fetch cycle, thereby reducing latency without requiring complex real-time prediction computation during the fetch operation.
3Productivity
If decode clusters are used for instruction decoding, then instruction execution throughput is improved, but load balancing difficulty increases
Solution Approach 1:
The patent implements feedback mechanisms that monitor the operational status and workload distribution across multiple decode clusters. Based on this feedback, the system dynamically adjusts instruction dispatching decisions to balance the load more effectively. The feedback loop allows the system to learn from past imbalances and make more informed routing decisions, improving throughput while managing load balancing complexity through adaptive control.
Data Source
AI summary
Techniques and mechanisms for providing branch prediction information to facilitate instruction decoding by a processor. In an embodiment, entries of a branch prediction table (BTB) each identify, for a corresponding instruction, whether a prediction based on the instruction (if any) is eligible to be communicated, with another prediction, in a single fetch cycle. A branch prediction unit of the processor determines a linear address of a fetch region which is under consideration, and performs a search of the BTB based on the linear address. A result of the search is evaluated to detect for any hit entry which indicates a double prediction eligibility. In another embodiment, where it is determined that double prediction eligibility is indicated for an earliest one the instructions represented by the hit entries, multiple predictions are communicated in a single fetch cycle.