Store Dependence Predictor Skip Me Bit Mechanism
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Solution Overview
Problem
Current processors rely on load-based dependence prediction, which can lead to performance bottlenecks due to conservative waiting for store address resolution, even when no conflict exists between store and load operations, limiting speculative execution and completion of loads.
Innovation Solution
Implementing a store dependence predictor (SDP) that sets a 'skip me' bit in the store buffer entries, allowing younger loads to speculatively execute ahead of stores without resolved addresses, and using a content addressable memory array to train and verify predictions based on load hits and misses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If load-based dependence prediction is used to allow speculative load execution, then load completion speed is improved, but false predictions cause incorrect data loading and performance degradation
Solution Approach 1:
The system implements feedback by having loads verify predictions against completed younger loads after speculative execution. This feedback mechanism allows the system to learn from past predictions and improve future accuracy while maintaining the speed benefit of speculative execution.
Solution Approach 2:
The store dependence predictor performs preliminary action by predicting store address resolution status before load execution. This allows loads to speculate ahead of stores without resolved addresses, improving load completion speed while the predictor continuously refines its accuracy through feedback.
2Reliability
If conservative waiting for store address resolution is implemented, then prediction reliability is improved, but load execution time increases and processor performance deteriorates
Solution Approach 1:
The store dependence predictor performs preliminary prediction of store address resolution status before loads need to wait for address resolution. This preliminary action allows loads to execute speculatively without conservative waiting, reducing execution time while maintaining reliability through continuous verification.
Solution Approach 2:
The system implements skipping by allowing loads to rush through stores without resolved addresses when the predictor indicates no conflict. This eliminates unnecessary waiting time while the feedback mechanism ensures correctness is maintained through verification with completed stores.
3Reliability
If store address resolution is required before load execution, then data correctness is ensured, but processor throughput decreases due to blocking
Solution Approach 1:
The store dependence predictor performs preliminary prediction before load execution, determining whether stores with unresolved addresses will conflict. This allows loads to execute without waiting for address resolution when safe, improving throughput while data correctness is maintained through verification with completed stores.
Solution Approach 2:
The system enables loads to skip over stores without resolved addresses when the predictor indicates no potential conflict. This rushing through mechanism eliminates blocking and improves processor throughput, while correctness is ensured through the feedback verification process with completed younger loads.
Data Source
AI summary
An apparatus and method for store dependence prediction is described. For example, one embodiment of the invention includes a processor comprising a store buffer for buffering store operations prior to completion, the store operations to store data to a memory hierarchy; and a store dependence predictor to predict whether load operations should be permitted to speculatively skip over each store operation and responsively setting an indication within an entry associated with each store operation in the store buffer; wherein a load operation checks the indication in the store buffer to determine whether to speculatively execute ahead of each store operation.