Branch Target Buffer Entry Allocation for Indirect Branch Prediction
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Solution Overview
Problem
Branch target prediction in processors is challenging due to the large number of possible valid target addresses, leading to inaccurate predictions and pipeline stalls, especially in cases of indirect branches.
Innovation Solution
A circuit and method are implemented where entries in a branch target buffer include a tag value, target value, and prediction accuracy value, with a control module selecting and allocating entries based on prediction accuracy to improve prediction accuracy by updating and managing entries accordingly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If branch target prediction is implemented to improve processor performance, then instruction-level parallelism is improved, but prediction accuracy deteriorates due to the large number of possible target addresses
Solution Approach 1:
The branch target buffer is divided into multiple sets, with each set containing multiple ways (e.g., 2-way, 4-way, 8-way set associative). This segmentation allows the system to manage a large number of target addresses by organizing them into manageable groups, improving both prediction accuracy and performance by enabling selective access to relevant prediction entries.
Solution Approach 2:
The patent implements dynamic selection of prediction entries based on prediction accuracy values. The control module dynamically determines which entry to allocate or use by evaluating accuracy metrics, allowing the system to adapt to changing prediction quality and maintain high accuracy despite the large address space.
2Measurement precision
If multiple prediction entries are maintained in the branch target buffer, then prediction accuracy is improved, but device complexity increases
Solution Approach 1:
The branch target buffer is segmented into multiple sets and ways, allowing prediction entries to be organized in a structured manner. This segmentation reduces complexity by providing a systematic method for managing multiple entries, enabling parallel comparison and selection processes.
Solution Approach 2:
The patent uses multiple ways within each set (e.g., 2-way, 4-way, 8-way) to provide redundant prediction entries. This partial excess allows the system to maintain multiple candidate predictions simultaneously, improving accuracy while managing complexity through organized parallel structures rather than unmanaged proliferation.
3Device complexity
If indirect branch instructions are handled with traditional prediction methods, then device complexity is kept low, but prediction accuracy deteriorates due to inaccurate predictions and pipeline stalls
Solution Approach 1:
The patent introduces an intermediary control module that manages the selection and allocation of prediction entries for indirect branches. This intermediary layer coordinates between the branch target buffer and the prediction logic, enabling accurate indirect branch handling through structured entry selection based on tag matching and accuracy evaluation.
Solution Approach 2:
The system dynamically adapts to indirect branch characteristics by evaluating prediction accuracy values and selectively allocating entries based on matching criteria. This dynamic approach allows the system to optimize for indirect branches without requiring a completely separate complex mechanism, maintaining flexibility and accuracy.
Data Source
AI summary
A circuit for implementing a branch target buffer. The branch target buffer may include a memory that stores a plurality of entries. Each entry may include a tag value, a target value, and a prediction accuracy value. A received index value corresponding to an indirect branch instruction may be used to select one of entries of the plurality of entries, and a received tag value may then be compared to the tag value of the selected entries in the memory. An entry in the memory may be selected in response to a determination that the received tag does not match the tag value of compared entries. The selected entry may be allocated to the indirect instruction branch dependent upon the prediction accuracy values of the plurality of entries.