Address-Based Filtering for Low-Power Load/Store Speculation
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Solution Overview
Problem
Content addressable memory (CAM) structures used for checking memory dependencies in load/store units are power-intensive, necessitating a more efficient method to reduce power consumption without performance degradation.
Innovation Solution
Implement a filtering mechanism that maintains a table of address ranges for previously received instructions, allowing certain instructions to bypass costly CAM lookups by tracking in-flight loads and stores, reducing the need for full or partial address comparisons.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If CAM structures are used to check memory dependencies in load/store units, then ordering violation detection is ensured, but power consumption increases
Solution Approach 1:
The patent segments the address comparison function into two parts: a filtering table that handles range-based preliminary filtering, and a CAM structure that handles precise address matching. This segmentation allows the system to avoid unnecessary CAM lookups for addresses that cannot possibly match, thereby reducing power consumption while maintaining detection reliability.
Solution Approach 2:
The filtering table performs preliminary address range checking before addresses are passed to the CAM structure. By pre-filtering addresses that fall outside relevant ranges, the system performs the useful action of eliminating non-matching cases in advance, reducing the workload and power consumption of the subsequent CAM lookup operations.
2Measurement precision
If full physical address comparisons are performed for every load/store instruction, then memory dependency accuracy is maintained, but processing time increases
Solution Approach 1:
The address comparison process is divided into two stages: first, a rapid range-based filter checks whether the address could potentially match; second, only addresses that pass the filter undergo precise CAM-based comparison. This segmentation eliminates time-wasting comparisons while preserving accuracy for relevant cases.
Solution Approach 2:
The filtering table performs preliminary range validation before full address comparison. Addresses that fail the preliminary range check are immediately discarded without undergoing time-consuming CAM lookups, thus reducing processing time while maintaining accuracy for addresses that require detailed comparison.
3Reliability
If CAM lookups are performed for all incoming instructions, then ordering checks are thorough, but power consumption and processing time increase
Solution Approach 1:
The instruction processing pipeline is segmented into two paths: a fast path through the filtering table for address range screening, and a slower path through the CAM structure for detailed dependency checking. This segmentation allows the majority of instructions to be processed quickly while maintaining thorough checking for those that require it.
Solution Approach 2:
The filtering table performs preliminary screening of all incoming instructions to identify only those that require detailed CAM lookup. By performing this preliminary action, the system avoids unnecessary thorough checks on instructions that cannot possibly have ordering violations, thereby improving throughput while maintaining check thoroughness for relevant cases.
Data Source
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AI summary
Address-based filtering for load/store speculation includes maintaining a filtering table including table entries associated with ranges of addresses; in response to receiving an ordering check triggering transaction, querying the filtering table using a target address of the ordering check triggering transaction to determine if an instruction dependent upon the ordering check triggering transaction has previously been generated a physical address; and in response to determining that the filtering table lacks an indication that the instruction dependent upon the ordering check triggering transaction has previously been generated a physical address, bypassing a lookup operation in an ordering violation memory structure to determine whether the instruction dependent upon the ordering check triggering transaction is currently in-flight.