Load Store Execution Unit Data Reuse Mechanism
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Solution Overview
Problem
Processing systems face high latency and power consumption due to repeated execution of load instructions, which incur additional cache access expenses and stall dependent instructions, highlighting the need to reduce these costs without compromising functional correctness.
Innovation Solution
A load store execution unit determines if a load instruction's data can be reused from a prior execution without accessing the data cache, checking for functional errors and ensuring no changes in the cache line, thereby reusing the data if no errors are found.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If load instructions are re-executed to ensure functional correctness, then reliability is improved, but latency and power consumption increase
Solution Approach 1:
The system performs preliminary checks before re-executing load instructions to determine if re-execution is necessary. The load store execution unit checks whether the cache line has been modified since the previous execution, and only re-executes if modifications are detected, thereby avoiding unnecessary re-execution latency while maintaining functional correctness
Solution Approach 2:
The system implements a feedback mechanism where the load store execution unit monitors cache line modification status and uses this information to control subsequent load instruction executions. This feedback loop allows the system to dynamically adjust re-execution behavior based on actual cache state, reducing unnecessary latency
2Reliability
If load instructions access the data cache for each execution, then data freshness is ensured, but power consumption increases
Solution Approach 1:
The system performs a preliminary check of the cache line modification status before accessing the data cache. The load store execution unit determines whether the cache line has been modified since the previous load execution, and only accesses the cache if modifications are detected, thereby saving power while ensuring data freshness when necessary
Solution Approach 2:
The system uses a copy of the modification status information (stored in the load store execution unit) to determine whether cache access is needed, rather than directly accessing the cache for every load instruction. This copying approach allows the system to avoid unnecessary cache accesses and reduce power consumption
3Reliability
If load instructions are stalled while waiting for cache access, then data accuracy is maintained, but productivity decreases
Solution Approach 1:
The system performs preliminary determination of cache line modification status before stalling instructions. The load store execution unit checks whether the cache line has been modified, and only stalls dependent instructions if modifications are detected and cache access is required. This approach maintains data accuracy while minimizing stalls and improving overall instruction throughput
4Reliability
If evaluation load instructions consume access ports during re-execution, then functional checks are completed, but other instructions are denied execution opportunities
Solution Approach 1:
The system performs preliminary determination of whether re-execution is necessary before consuming access ports. The load store execution unit checks cache line modification status and only initiates re-execution (which would consume access ports) if modifications are detected. This reduces unnecessary port consumption and allows other instructions to execute, improving overall throughput while maintaining functional correctness
Data Source
AI summary
Systems and methods for reusing load instructions by a processor without accessing a data cache include a load store execution unit (LSU) of the processor, the LSU being configured to determine if a prior execution of a first load instruction loaded data from a first cache line of the data cache and determine if a current execution of the second load instruction will load the data from the first cache line of the data cache. Further, the LSU also determines if a reuse of the data from the prior execution of the first load instruction for the current execution of the second load instruction will lead to functional errors. If there are no functional errors, the data from the prior execution of the first load instruction is reused for the current execution of the second load instruction, without accessing the data cache for the current execution of the second load instruction.