Load-Store Collision Detection in Speculative Out-of-Order Engines
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Solution Overview
Problem
In speculative out-of-order processing engines, load-store collisions lead to pipeline flushing and re-dispatching of instructions, resulting in significant performance penalties due to incorrect speculative dispatch of load and store instructions.
Innovation Solution
A load-store collision detection system that includes a load pipeline, scheduler, and load-store queue, which compares load instruction virtual addresses with store instruction virtual addresses to assert a clear signal when a match occurs, preventing premature dispatch of dependent instructions and minimizing pipeline flushing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If load and store instructions are speculatively dispatched without address collision detection, then instruction dispatch throughput is improved, but pipeline flushing and re-dispatch penalties increase when collisions occur
Solution Approach 1:
The patent applies preliminary action by detecting potential load-store address collisions before the load instruction completes execution. The load-store queue compares the load instruction's virtual address with store instruction virtual addresses early in the pipeline, allowing the system to preemptively invalidate speculatively dispatched load instructions and prevent pipeline flushing that would occur if collisions were detected later.
2Reliability
If load-store collision detection is implemented, then pipeline flushing is reduced, but device complexity increases due to additional queue and comparison logic
Solution Approach 1:
The load-store queue serves multiple functions: it stores virtual addresses of in-flight store instructions, performs address collision detection with load instructions, and provides invalidation signals to the scheduler. This multi-functionality reduces the need for separate dedicated components for each function, thereby mitigating the increase in device complexity while improving speculative dispatch accuracy.
3Device complexity
If load instructions are validated only at execution stage, then scheduler simplicity is maintained, but premature dispatch of dependent instructions occurs causing performance penalties
Solution Approach 1:
The patent implements feedback by having the load-store queue continuously monitor and compare addresses, then provide invalidation signals back to the scheduler when potential collisions are detected. This feedback mechanism allows the scheduler to adjust its dispatch decisions in real-time, preventing premature dispatch of dependent instructions while maintaining relatively simple scheduler structure.
Data Source
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AI summary
A load-store collision detection system for a speculative out of order processing engine which includes a scheduler that dispatches instructions to multiple instruction pipelines. The instruction pipelines include a load pipeline that provides a load valid signal when a speculatively dispatched load instruction is executing. The load-store collision detection system includes comparator logic, broadcast logic, and kill logic. The comparator logic asserts a clear signal when a virtual address of the speculatively dispatched load instruction matches at least one store instruction virtual address of a previously dispatched store instruction whose corresponding store data is not ready yet. The broadcast logic broadcasts the load valid signal to the scheduler to enable dispatch of any instructions dependent upon the speculatively dispatched load instruction. The kill logic invalidates the load valid signal when the clear signal is asserted to avoid a load-store collision that reduces processing performance.