Speculative Load Address Prediction Circuitry for Processor Latency Reduction
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Solution Overview
Problem
Load instructions in processing systems often result in significant latency, leading to performance issues due to varying access times depending on data location, with subsequent instructions stalling as a result, especially when source operands are not immediately available.
Innovation Solution
An interconnect apparatus with decode and prediction circuitry that predicts the target address of load instructions using a prediction scheme, triggering speculative memory access to reduce latency and improve processor performance by caching data before it is needed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor waits for load instructions to complete before executing subsequent instructions, then data accuracy is ensured, but processor performance deteriorates due to pipeline stalls
Solution Approach 1:
The patent performs preliminary actions by speculatively executing load instructions before the target address is fully resolved. The prediction circuitry predicts target addresses and initiates memory accesses in advance, allowing data to be ready when needed. This preliminary action resolves the contradiction by preparing data beforehand without waiting for complete address resolution, thus maintaining accuracy while improving performance.
Solution Approach 2:
The patent implements feedback mechanisms where the prediction circuitry continuously refines its predictions based on actual target addresses and access patterns. When speculative loads are validated or corrected, this information feeds back to improve future predictions. This feedback loop ensures data accuracy is maintained while enabling aggressive speculation to improve processor performance.
2Reliability
If the processor stalls subsequent instructions during load operations, then correct data is ensured to be loaded, but latency increases significantly
Solution Approach 1:
The system performs preliminary memory access operations by predicting target addresses and initiating loads before the actual execution point. This allows the correct data to be fetched in advance, eliminating the need for stalls while ensuring data correctness through validation mechanisms that compare predicted versus actual addresses.
Solution Approach 2:
The patent introduces dynamic behavior by making the load execution flexible - sometimes executing speculatively ahead of time, sometimes waiting for address resolution, and sometimes correcting mispredictions. This dynamic approach optimizes the balance between ensuring correct data loading and minimizing latency based on runtime conditions.
3Loss of time
If speculative memory access is performed, then latency is reduced by pre-loading data, but complexity increases due to prediction circuitry
Solution Approach 1:
The patent segments the address generation and memory access functions into separate components: prediction circuitry for address prediction, validation logic for verifying predictions, and execution units for actual load operations. This segmentation allows each component to be optimized independently, reducing overall complexity while enabling speculative execution to reduce latency.
Solution Approach 2:
The patent introduces intermediary structures such as prediction buffers and validation stages that mediate between the prediction circuitry and the execution units. These intermediaries simplify the interaction between components, managing the complexity of speculative execution while maintaining the latency reduction benefits.
4Device complexity
If load instructions are executed without prediction, then device complexity is minimized, but processor performance suffers due to pipeline stalls
Solution Approach 1:
By performing preliminary address prediction and initiating memory accesses before the load instruction fully executes, the system eliminates pipeline stalls without requiring complex restructuring of the execution pipeline. The prediction happens in parallel with instruction decoding, maintaining simplicity while improving performance.
Solution Approach 2:
The patent ensures continuity of useful action by keeping the pipeline full through speculative execution. While simple execution without prediction would stall the pipeline waiting for memory operations, the prediction mechanism maintains continuous execution flow, improving performance without significantly increasing complexity.
Data Source
AI summary
Aspects of the present disclosure relate to an apparatus comprising decode circuitry to receive an instruction and identify the received instruction as a load instruction, and prediction circuitry to predict, based on a prediction scheme, a target address of the load instruction, and trigger a speculative memory access in respect of the predicted target address.