Speculative Load Reissue for Memory Ordering Compliance
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Solution Overview
Problem
Data processing systems face challenges in maintaining memory ordering requirements when value predictions are used, as they can lead to memory ordering violations even if the prediction is correct, due to potential changes in target data between the time of prediction and actual loading, causing inconsistencies and incorrect executions.
Innovation Solution
Incorporating load tracking circuitry to maintain tracking information for load operations and predict speculative load operations, re-issuing operations when there's a risk of target data change, independent of the prediction's correctness, to ensure memory ordering compliance, even if the addresses do not match.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If value prediction is used to speculatively issue load operations ahead of older load operations, then productivity is improved by enabling earlier execution, but reliability deteriorates due to potential memory ordering violations when target data changes between prediction and actual loading
Solution Approach 1:
The system implements a feedback mechanism where load tracking circuitry monitors target data for speculatively issued load operations and compares it with data loaded by older load operations. When a mismatch is detected indicating a memory ordering violation, the system triggers re-issuing of the speculatively issued load operation, thereby correcting the error while maintaining overall system reliability.
Solution Approach 2:
The system performs preliminary value prediction to enable speculative issuance of load operations before actual data loading completes. This preliminary action allows the processor to proceed with dependent operations earlier, improving productivity while the load tracking circuitry subsequently verifies correctness to maintain reliability.
2Reliability
If load tracking circuitry monitors all load operations to detect memory ordering violations, then reliability is improved by ensuring correct execution, but device complexity increases due to additional tracking and comparison mechanisms
Solution Approach 1:
The load tracking circuitry applies monitoring selectively rather than universally. It specifically tracks speculatively issued load operations that have bypassed older load operations, focusing resources on the critical cases where memory ordering violations can occur, rather than monitoring all load operations equally.
Solution Approach 2:
The system segments the load operation monitoring into distinct components: identification of speculative loads, tracking of target data, comparison with older load operations, and conditional re-issuing. This segmentation allows each component to be optimized independently and reduces overall complexity compared to a monolithic tracking approach.
Data Source
AI summary
An apparatus has processing circuitry, load tracking circuitry and load prediction circuitry. It is determined whether tracking information indicates that there is a risk of target data, corresponding to an address of a speculatively-issued load operation which is speculatively issued (bypassing an older operation) based on a prediction determined by the load prediction circuitry, having changed between the target data being loaded for the speculatively-issued load operation and data being loaded for a given older load operation bypassed by the speculatively-issued load operation. If so, independent of whether the addresses of the speculatively-issued load operation and the given older load operation correspond, at least the speculatively-issued load operation is reissued, even when the prediction is correct. This protects against ordering violations.