Register Renaming via Partial Flag Granule Segmentation
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Solution Overview
Problem
In out-of-order data processing systems, the short dependency chain between instructions due to partial updates of data granules, such as flag registers, hinders rapid operation and efficient scheduling, as most instructions require the most recent source register values, leading to delayed instruction issuance.
Innovation Solution
The use of multiple tag values associated with data granules to track writes to different subsets of values, allowing instructions to draw non-overwritten portions from the oldest available source registers, thereby lengthening the dependency chain and reducing delays in issuing instructions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If instructions wait for the most recent source register values to be available before issuing, then data dependency accuracy is maintained, but instruction issuance is delayed reducing productivity
Solution Approach 1:
The data granule is segmented into multiple portions, each tracked by a separate tag value. This allows the system to identify which specific portions have been updated and which remain valid in older registers, enabling selective reuse of non-overwritten portions without waiting for complete updates.
Solution Approach 2:
Different portions of the data granule have different update states - some portions may be overwritten while others remain valid. The tag values track these local quality differences, allowing instructions to draw from the oldest available source registers for portions that haven't been overwritten, rather than requiring the entire granule to be updated.
2Device complexity
If the dependency chain between instructions is kept short by using only the most recent source register values, then scheduling simplicity is maintained, but instruction parallelisation is reduced
Solution Approach 1:
The rename table is segmented into multiple tag value entries (first, second, third tag values) that track different portions of the data granule. This segmentation allows the scheduling mechanism to selectively extend dependency chains only where necessary for overwritten portions while maintaining short dependencies for non-overwritten portions.
Solution Approach 2:
The system adds a temporal dimension to register tracking by maintaining multiple tag values that represent different points in the update history. This allows the system to look back in time to find valid source register values from earlier updates, effectively extending the dependency chain in the time dimension without increasing scheduling complexity.
3Loss of time
If multiple tag values are used to track different portions of data granules, then instruction issuance delay is reduced, but device complexity increases
Solution Approach 1:
The data granule is divided into multiple trackable portions with associated tag values. This segmentation enables the system to reduce instruction issuance delay by identifying valid source registers for each portion independently, while the complexity is managed through a systematic rename table structure that maps architectural registers to virtual registers and their corresponding tag values.
Data Source
AI summary
A processor 2 supporting register renaming has a rename table 20 in which the flag register has multiple tag values associated therewith. These tag values indicate which virtual register corresponds to a destination flag register of the oldest instruction which wrote a still up-to-date value of a subset of the flags.