Sampled Register Branch Prediction for Fewer Pipeline Flushes
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Solution Overview
Problem
Existing branch prediction mechanisms struggle with accuracy due to the significant variation in register operand values between the time of prediction and execution, leading to frequent pipeline flushes and performance degradation.
Innovation Solution
Implementing sampled state storage to store register operand values at a sampling point and using prediction circuitry to determine branch outcomes based on these sampled values when a semantic branch condition is satisfied, triggered by events such as misprediction or specific decoding, allowing for accurate predictions of future branch instructions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If traditional branch prediction mechanisms are used, then the system can execute instructions speculatively without waiting for branch outcomes, but prediction accuracy deteriorates due to register operand value variation between prediction and execution
Solution Approach 1:
The patent stores sampled register operand values at a sampling point in program flow before the branch instruction is executed. This preliminary action captures the actual register values that will be used at execution time, allowing the branch prediction to be based on these pre-captured values rather than current values, thereby resolving the accuracy issue while maintaining speculative execution speed
Solution Approach 2:
The patent creates a copy of the register operand values by sampling and storing them in sampled state storage. This copy is then used by the prediction circuitry to determine branch outcomes, eliminating the problem of value variation between prediction and execution without requiring the actual execution to wait for updated values
2Reliability
If prediction accuracy is improved by using sampled register values, then pipeline flushes are reduced, but device complexity increases due to additional sampled state storage and prediction circuitry
Solution Approach 1:
The sampled state storage and prediction circuitry can serve multiple branch instructions and different program flows. The sampling mechanism is triggered by semantic branch conditions that can apply to various types of branch instructions, making the additional hardware components universally applicable rather than dedicated to a single function, thereby reducing the relative complexity overhead
3Productivity
If speculative execution is enabled to improve performance, then instruction throughput increases, but loss of time occurs due to pipeline flushes when predictions are incorrect
Solution Approach 1:
The patent uses feedback from the actual execution path to validate or correct predictions. By comparing the predicted branch outcome with the actual outcome determined during execution, the system can learn from mispredictions and adjust future predictions, reducing the frequency of pipeline flushes while maintaining high instruction throughput through speculative execution
Data Source
AI summary
An apparatus comprises sampled state storage to store sampled register values of a register operand sampled at a sampling point in program flow, and prediction circuitry. In response to a semantic branch trigger indicating that there is a future branch instruction at a point in program flow later than the sampling point which satisfies a semantic branch condition, the prediction circuitry is configured to make a determination of a branch outcome of the future branch instruction based on a sampled register value of a particular register operand. The semantic branch condition is satisfied by a given future branch instruction for which a branch outcome is dependent on a sampled register value stored in the sampled state storage, and a value that the given register operand will have when program flow reaches the given future branch instruction can be calculated deterministically based on the sampled register value of the given register operand.