Fast Index Tree Accelerates Branch Prediction
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Solution Overview
Problem
Existing branch prediction techniques in computer processing systems face inefficiencies due to pipeline stalls caused by conditional branch instructions, leading to increased latency and reduced throughput, as they often require multiple cycles to resolve branch targets, especially when auxiliary structures like pattern history tables and multiple target tables are not needed.
Innovation Solution
A fast index tree (FIT) structure is introduced to accelerate branch prediction by re-indexing the branch target buffer (BTB) search, using a content addressable memory to translate between BTB and FIT locations, and redirecting searches based on taken and not-taken branch paths, thereby reducing the number of cycles required for prediction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional branch prediction methods are used, then branch target prediction can be performed, but pipeline stalls occur due to multiple cycles required to resolve branch targets
Solution Approach 1:
The patent applies preliminary action by pre-computing and storing branch target predictions in the branch target buffer (BTB) before the branch instruction is executed. The Fast Index Tree (FIT) structure pre-calculates next search addresses based on taken and not-taken paths, allowing the BTB search to skip unnecessary cycles and achieve faster branch target resolution without waiting for execution stage completion.
Solution Approach 2:
The patent introduces the Fast Index Tree (FIT) structure as an intermediary between the branch instruction and the BTB search. The FIT structure takes the branch address and predicts the next search address in the BTB, acting as a mediator that accelerates the re-indexing process and reduces the number of cycles required to locate branch target predictions.
2Reliability
If auxiliary structures like pattern history tables and multiple target tables are used, then prediction accuracy may improve, but device complexity and power consumption increase
Solution Approach 1:
The patent applies the extraction principle by removing unnecessary auxiliary structures (pattern history tables and multiple target tables) from the branch prediction system. The invention demonstrates that the BTB combined with the FIT structure can achieve sufficient prediction accuracy without these additional components, thereby simplifying the device architecture and reducing power consumption while maintaining effective branch prediction.
Data Source
AI summary
Embodiments relate to using a fast index tree for accelerated branch prediction. A system includes a branch target buffer, a FIT structure, and a processing circuit configured to perform a method. The method includes determining that searching of the branch target buffer is to be performed under FIT control. A current search address for searching of the branch target buffer is saved. The branch target buffer is searched at the saved current search address for a branch prediction. A FIT next-search address is determined based on reading branch taken and branch not taken paths for a next search level of predicted branches from the FIT structure. The searching of the branch target buffer is re-indexed based on the FIT next-search address. It is determined whether the searching at the saved current search address located the branch prediction.