Speculative Execution State Management via Selective Storage
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Solution Overview
Problem
Speculative execution in data processing leads to high costs due to the need for rewinding instructions when predictions are incorrect, as existing methods do not efficiently manage the state changes and storage of previous states.
Innovation Solution
A data processing apparatus with processing circuitry that speculatively executes instructions, storage circuitry to store current and previous states, and flush circuitry to replace the current state with a previous state upon prediction miss, reducing the extent of rewinding required.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If speculative execution is used to continue executing instructions before control flow result is known, then execution speed is improved, but the cost of rewinding on miss-prediction becomes high
Solution Approach 1:
The patent stores multiple previous states of the processing circuitry in advance (before they are needed for rewinding). When a miss-prediction occurs, the system can immediately restore from a pre-stored state rather than performing expensive rewinding operations. This preliminary storage of states eliminates the need for costly backward execution and state restoration.
2Loss of energy
If multiple previous states are stored to reduce rewinding cost, then processing resources during rewinding are minimized, but storage requirements increase
Solution Approach 1:
The patent selectively stores previous states based on their usefulness for potential rewinding operations. Rather than storing all possible states uniformly, the system identifies and stores only those states that are likely to be needed (recent states that correspond to plausible alternative execution paths). This selective storage reduces the overall storage burden while maintaining the ability to perform efficient rewinding when needed.
Data Source
AI summary
A data processing apparatus is provided. It includes processing circuitry for speculatively executing a plurality of instructions. Storage circuitry stores a current state of the processing circuitry and a plurality of previous states of the processing circuitry. Execution of the plurality of instructions changes the current state of the processing circuitry. Flush circuitry replaces, in response to a miss-prediction, the current state of the processing circuitry with a replacement one of the plurality of previous states of the processing circuitry.