Trace Checkpoint Rollback for Memory Ordering Violations
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Solution Overview
Problem
Current processor architectures face challenges in efficiently and speculatively executing traces and memory operations while reducing hardware and power consumption, particularly in maintaining memory coherency and handling out-of-order execution of instructions like load and store operations.
Innovation Solution
A circuit is introduced that includes a trace unit and an execution unit, with a memory ordering protocol that sequences traces, caches data, and uses checkpoint entries to manage memory operations, detecting and resolving ordering violations to ensure program order constraints are maintained, allowing for efficient execution and reduced hardware complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If speculative execution of traces is enabled to improve performance, then productivity increases, but memory coherency and ordering constraints become difficult to maintain
Solution Approach 1:
The patent applies preliminary action by creating checkpoint entries before speculative trace execution. These checkpoints record the state of memory operations and ordering constraints prior to speculative changes, enabling the system to quickly restore to a known good state if the speculative execution fails or violates coherency rules.
Solution Approach 2:
The patent implements feedback mechanisms through memory ordering violators that detect when speculative execution violates ordering constraints. When violations are detected, the system generates rollback signals that feed back into the execution pipeline, triggering corrective actions to restore proper ordering and coherency.
2Productivity
If out-of-order execution is used to improve performance, then productivity increases, but device complexity increases due to additional hardware needed for ordering management
Solution Approach 1:
The patent uses copying by creating checkpoint entries that are copies of the pre-execution state. Instead of complex real-time tracking of all ordering constraints, the system makes copies of the state before speculative execution and uses these copies for rollback purposes, simplifying the hardware needed for ordering management.
Solution Approach 2:
The patent applies segmentation by dividing the memory operation management into discrete segments: checkpoint entries for different memory operations, ordering constraint trackers, and rollback mechanisms. This modular segmentation reduces overall hardware complexity by making each component independent and manageable.
3Productivity
If trace cache management is optimized for speculative execution, then productivity improves, but loss of time increases due to rollbacks on failed traces
Solution Approach 1:
The patent applies preliminary action by preparing checkpoint entries in advance before speculative trace execution begins. This pre-preparation of rollback information eliminates the need for complex real-time rollback calculations, reducing the time penalty when rollbacks are necessary by having the restoration data already ready.
Data Source
AI summary
This invention includes a circuit for tracking memory operations with trace-based execution. Each trace includes a sequence of operations that includes zero or more of the memory operations. The memory operations being executed form a set of active memory operations that have a predefined program order among them and corresponding ordering constraints. At least some of the active memory operations access the memory in an execution order that is different from the program order. Checkpoint entries are associated with each trace. Each entry refers to a checkpoint location. Memory operation ordering entries correspond to each one of the active memory operations. Violations of the ordering constraints result in overwriting the checkpoint locations associated with the selected trace as well as the checkpoint locations associated with traces that are younger than the selected trace.