Register Readiness Checks for Value Prediction Latency
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Solution Overview
Problem
Value prediction techniques in processor design are limited by the time-consuming lookup for previously computed results, which can cause significant latency due to cache misses, making them impractical for functions beyond the largest ones.
Innovation Solution
The system employs a Branch-Register-Not-Ready (BRNR) instruction to check the readiness of a destination register, allowing for the use of previously computed results without unnecessary recalculation, and includes a lookup table to store results and inputs, with variations that include delayed checks and multiple BRNR instructions to ensure accurate and efficient value prediction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If value prediction is implemented using a lookup table, then previously computed results can be reused to avoid recalculation, but the lookup process may cause significant latency due to cache misses
Solution Approach 1:
The system performs preliminary actions by checking register readiness status before initiating the lookup process. The BRNR instruction checks whether the destination register is ready to receive data, and only if it is ready does the system proceed with the lookup table access. This preliminary check prevents initiating time-consuming cache lookups when the result will not be used, thereby reducing lookup latency while maintaining value prediction effectiveness.
2Loss of time
If the system checks register readiness using BRNR instruction before lookup, then unnecessary cache lookups are avoided, but the system requires additional instruction overhead
Solution Approach 1:
The BRNR instruction serves as an intermediary between the value prediction logic and the lookup table access. It mediates the decision-making process by providing a simple readiness check that determines whether subsequent lookup operations should proceed. This intermediary instruction adds minimal complexity compared to the alternative of always performing full cache lookups, which would be significantly more time-consuming.
3Adaptability or versatility
If the system always performs lookup before function execution, then value prediction can be applied to all functions, but processor performance decreases due to cache miss latency
Solution Approach 1:
The system applies local quality by making the value prediction mechanism context-dependent rather than universally applied. The BRNR instruction checks the local state of the destination register, and value prediction is only activated when the register is ready. This localized approach ensures that value prediction is applied selectively to cases where it will actually improve performance, rather than being forced onto all functions regardless of benefit, thus maintaining both versatility and processor performance.
Data Source
AI summary
One embodiment of the present invention provides a system for using register readiness to facilitate value prediction. The system starts by loading a previously computed result for a function to a destination register for the function from a lookup table. The system then checks the destination register for the function by using a Branch-Register-Not-Ready (BRNR) instruction to check the readiness of the destination register. If the destination register is ready, the system uses the previously computed result in the destination register as the result of the function. Loading the value from the lookup table in this way avoids unnecessarily calculating the result of the function when that result has previously been computed.