Processor Pipeline Removal of Dependent Instructions
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Solution Overview
Problem
External memory accesses in processors can lead to stall conditions, particularly when subsequent instructions are dependent on the results of these accesses, which can hinder performance and reduce the benefits of multithreaded processing.
Innovation Solution
A processor implementation that stores an indication of external memory accesses in a data storage circuit, allowing dependent instructions to be removed from the execution pipeline, enabling non-dependent instructions from other threads to proceed, thereby maintaining in-order execution and reducing idle cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor waits for external memory access to complete before executing subsequent dependent instructions, then instruction correctness is maintained, but processor performance deteriorates due to pipeline stalls
Solution Approach 1:
The processor speculatively executes subsequent instructions before the external memory access completes by predicting that the access will not be needed. If the prediction is correct, the instructions are already in the pipeline ready for execution. If the prediction is wrong, the instructions are simply discarded when the memory access completes, avoiding pipeline stalls while maintaining correctness.
Solution Approach 2:
The processor dynamically adjusts pipeline behavior based on memory access patterns and instruction dependencies. The system monitors whether subsequent instructions are dependent on external memory accesses and adaptively decides whether to stall or discard instructions, optimizing performance for different workload characteristics while ensuring correctness when dependencies exist.
2Productivity
If the processor discards subsequent dependent instructions to avoid stalls, then processor performance improves, but instruction correctness deteriorates
Solution Approach 1:
The processor implements feedback mechanisms that monitor memory access completion status and instruction dependency relationships. When an external memory access completes, the system checks whether any discarded instructions were actually dependent on the accessed data. This feedback loop ensures that instructions are only discarded when safe, maintaining correctness while enabling performance optimizations.
Solution Approach 2:
The processor introduces intermediary structures such as dependency tracking circuits and instruction validation mechanisms that mediate between the speculative instruction execution and the actual memory access results. These intermediaries verify whether discarded instructions were truly independent, providing a safety layer that prevents correctness errors while allowing aggressive performance optimizations.
3Reliability
If the processor stalls the execution pipeline during external memory access, then dependent instruction correctness is ensured, but multithreaded processing benefits are reduced
Solution Approach 1:
The processor segments the instruction stream into independent and dependent instruction groups. When an external memory access occurs, only instructions that are proven to be dependent on the access result are stalled or discarded, while independent instructions from the same or different threads continue execution. This segmentation allows selective pipeline management that preserves multithreaded benefits while ensuring dependent instruction correctness.
Solution Approach 2:
The processor changes the execution parameter of pipeline stages based on instruction dependency analysis. For independent instructions, the pipeline continues normal operation with full throughput. For dependent instructions, the pipeline applies stall or discard operations. This dynamic parameter adjustment enables the system to maintain multithreaded processing efficiency while ensuring correctness for dependent instructions.
Data Source
AI summary
Techniques are disclosed relating to an apparatus, including a data storage circuit having a plurality of entries, and a load-store pipeline configured to allocate an entry in the data storage circuit in response to a determination that a first instruction includes an access to an external memory circuit. The apparatus further includes an execution pipeline configured to make a determination, while performing a second instruction and using the entry in the data storage circuit, that the second instruction uses a result of the first instruction, and cease performance of the second instruction in response to the determination.