Checkpointed Early Load Retirement for Processor Stall Reduction
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Solution Overview
Problem
Modern processors face performance issues due to long-latency loads, which block instruction retirement and resource usage, limiting the effectiveness of conventional load-value prediction as these loads clog the processor even after prediction, especially when they miss in the cache hierarchy.
Innovation Solution
The technique of checkpointed early load retirement, which involves predicting the outcome of long-latency loads, early retiring them, and using checkpoints to roll back and refetch in case of misprediction, allowing subsequent instructions to execute sooner and ensuring data integrity by buffering memory updates until validation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional load-value prediction is used to supply predicted values for unresolved loads, then dependent instructions can execute using the prediction, but the processor still clogs at retirement because the load must be compared against the predicted value, eventually blocking the processor to a stall
Solution Approach 1:
The patent extracts the comparison operation between the loaded value and predicted value from the retirement stage and moves it to the load completion stage. By performing the comparison early when the load returns from memory (rather than waiting for retirement), the system eliminates the bottleneck where unresolved loads clog the retirement pipeline, allowing subsequent instructions to retire without waiting for value verification
Solution Approach 2:
The patent performs the value comparison and retirement decision-making in advance before the traditional retirement stage. When a load returns from memory, the system immediately compares the returned value with the predicted value and makes the retirement decision at that point, rather than waiting for the instruction to reach the retirement stage, thereby preventing processor stalls
2Reliability
If unresolved long-latency loads remain at the processor's retirement stage for many cycles, then precise bookkeeping of architectural state is maintained, but processor performance is hindered due to blocked instruction retirement and resource holding
Solution Approach 1:
The patent performs the architectural state update in advance by committing register updates when the load returns from memory and the value is verified, rather than waiting for traditional retirement. This preliminary commitment maintains architectural state accuracy while enabling faster instruction retirement rates, as subsequent instructions can retire without waiting for long-latency load completion
Solution Approach 2:
The patent introduces dynamic retirement modes that adapt to load latency conditions. The system can operate in normal retirement mode for typical loads and switch to early retirement mode for long-latency loads, allowing the retirement mechanism to flex between precision mode (traditional in-order retirement) and performance mode (early retirement with comparison), thereby resolving the contradiction between reliability and productivity
Data Source
AI summary
A technique known as checkpointed early load retirement, combines register checkpointing load-value prediction to manage long-latency loads. When a long-latency load reaches the retirement stage unresolved, the processor enters Clear mode by (1) taking a Checkpoint of the architectural registers, (2) supplying a load-value prediction to consumers, and (3) early-retiring the long-latency load. This unclogs retirement, thereby “clearing the way” for subsequent instructions to retire, and also allowing instructions dependent on the long-latency load to execute sooner. When the actual value returns from memory, it is compared against the prediction. A misprediction causes the processor to roll back to the checkpoint, discarding all subsequent computation.