Load Store Ordering Violation Replay via Shadow Kill Window
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Solution Overview
Problem
Current processors face performance and power efficiency issues due to load/store ordering violations, which lead to costly flushes and power consumption, as predictors are finite and cannot prevent initial violations effectively.
Innovation Solution
A processor with a load/store unit that includes load and store pipelines, allowing load operations to be issued out of order, with a mechanism to detect and replay load operations via a shadow kill window, reducing the need for costly flushes by comparing store addresses with load addresses to prevent ordering violations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If load operations are executed out of order with respect to older store operations, then processor performance is improved, but load/store ordering violations occur causing flushes that reduce performance and increase power consumption
Solution Approach 1:
The patent applies preliminary action by detecting potential load/store ordering violations before they cause flushes. The shadow kill window mechanism proactively identifies when a load operation might violate ordering constraints with older store operations, and preemptively handles these cases by replaying the load instead of flushing, thus preventing the harmful effect while maintaining out-of-order execution performance benefits
2Reliability
If a predictor is used to predict load/store ordering violations, then some violations are avoided, but initial instances of violations cannot be avoided and the finite predictor requires retraining
Solution Approach 1:
The patent introduces an intermediary mechanism - the shadow kill window - that acts as a mediator between the out-of-order execution engine and the ordering constraints. This intermediary detects potential violations before they manifest as flushes, providing an additional layer of protection beyond the finite predictor and eliminating the need for predictor retraining while maintaining high reliability
3Reliability
If load operations are flushed when ordering violations occur, then correct operation is maintained, but performance and power efficiency are reduced
Solution Approach 1:
The patent converts the harmful effect of flushes into a beneficial replay mechanism. Instead of flushing and discarding speculatively generated results when ordering violations are detected, the system replays the load operation, thereby maintaining correct operation while preserving the computational work already done, thus improving both performance and power efficiency
Data Source
AI summary
A processor includes a load/store unit that includes one or more load pipelines and one or more store pipelines. Load operations may be issued into the load pipelines out of order with respect to older store operations. If a load operation is executed out or order with an older store operation that writes one or more bytes read by the load operation, and if the store operation is issued shortly after the load operation, such that the load operation is still in the load pipeline when the store operation is issued, some cases of flushing may be converted to replays by detecting the ordering violation while the load operation is still in the load pipeline.