Store Queue Store-to-Load Forward Verification
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Solution Overview
Problem
Processors face performance issues due to mistaken store-to-load forwarding, leading to incorrect data being forwarded to load instructions, which can result in catastrophic errors and unnecessary resynchronization operations.
Innovation Solution
A processor confirms the correctness of store-to-load forwarding operations by comparing dependent load instructions to both retired and unretired store instructions in the store queue, extending the time available for store operation confirmation and reducing unnecessary resynchronizations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor takes a conservative approach and performs resync operations when store-to-load forward correctness cannot be verified, then reliability is improved, but processor performance deteriorates
Solution Approach 1:
The processor performs preliminary verification of store-to-load forward correctness by comparing the load address with store addresses from retired store instructions before executing the load instruction. This preliminary action prevents incorrect data forwarding while allowing correct forwards to proceed without resync, thus maintaining both reliability and performance
Solution Approach 2:
The processor implements a feedback mechanism where the load/store unit verifies store-to-load forward operations by checking address matches between load instructions and retired store instructions. This feedback loop ensures correctness verification while providing a pathway to avoid unnecessary resync operations, balancing reliability and performance
2Manufacturing precision
If the processor extends the time available for store operation confirmation by maintaining store addresses after retirement, then manufacturing precision is improved, but device complexity increases
Solution Approach 1:
The store queue structure is enhanced to serve multiple functions: it not only tracks unretired store instructions but also maintains addresses of retired store instructions for verification purposes. This multi-functionality allows the same hardware structure to support both execution tracking and correctness verification, improving accuracy without proportionally increasing complexity
Solution Approach 2:
The verification mechanism adds a new dimension to the store queue functionality by incorporating temporal extension - maintaining store address information beyond the traditional retirement point. This dimensional change allows verification to occur at multiple time points (before and after retirement), improving verification accuracy while using existing hardware resources
Data Source
AI summary
A processor includes a store queue that stores information representing store instructions. In response to retirement of a store instruction, the processor invalidates the corresponding entry in the store queue, thereby indicating that the entry is available to store a subsequent store instruction. The store address is not removed from the queue until the subsequent store instruction is stored. Accordingly, the store address is available for comparison to a dependent load address.