Register Renaming Cache to Reduce RAT Complexity and Power
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Solution Overview
Problem
Register renaming techniques in processors face increasing complexity and power consumption issues due to high allocation widths, leading to power hotspots and timing criticality, especially with the use of register alias tables (RATs), which affect core frequency and performance.
Innovation Solution
Implement a register renaming cache that stores register renaming information for sequences of instructions between taken branches, reducing reliance on complex RATs by learning and caching renaming properties across instruction traces, allowing for efficient replay from the cache during subsequent executions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If register alias tables (RATs) are used for register renaming, then register renaming functionality is achieved, but complexity and power consumption increase non-linearly with allocation width
Solution Approach 1:
The patent segments the register renaming functionality by separating the RAT into multiple smaller tables (RAT0, RAT1, RAT2, etc.) that handle different portions of the allocation width. Each table manages a subset of physical registers, reducing the complexity of individual tables while collectively providing the full renaming capability. This segmentation allows the system to achieve high allocation width functionality without any single table becoming prohibitively complex.
Solution Approach 2:
The patent introduces a temporal dimension to register renaming by implementing a register renaming cache that stores renaming information from previously executed instruction traces. Instead of relying solely on complex RAT lookups for every instruction, the system caches renaming mappings and replays them for subsequent executions of the same trace, effectively moving some renaming operations to a different temporal dimension and reducing real-time complexity requirements.
2Adaptability or versatility
If register alias tables (RATs) are used for register renaming, then register renaming functionality is achieved, but power consumption increases and creates power hotspots
Solution Approach 1:
By segmenting the RAT into multiple smaller tables, the patent reduces the power consumption of individual tables. Each smaller table requires less switching activity and consumes less dynamic power, while the collective power consumption of all tables is distributed across the chip, preventing power hotspots from concentrating in a single large RAT structure.
Solution Approach 2:
The register renaming cache performs preliminary action by pre-computing and storing renaming mappings during the first execution of an instruction trace. When the same trace is executed again, the pre-computed mappings are replayed from the cache, avoiding the need to re-perform complex RAT lookups and reducing real-time power consumption during subsequent executions of frequently repeated code patterns.
3Adaptability or versatility
If register alias tables (RATs) are used for register renaming, then register renaming functionality is achieved, but timing criticality increases and affects core frequency
Solution Approach 1:
Segmenting the RAT into multiple smaller tables reduces the timing criticality of individual table lookups. Smaller tables have shorter access times and less complex decode logic, allowing the register renaming stage to complete faster and not become a bottleneck for core frequency. The segmented approach distributes the timing requirements across multiple simpler operations rather than one complex operation.
Solution Approach 2:
The register renaming cache performs preliminary computation of renaming mappings during the first trace execution and stores them for rapid replay. This preliminary action moves the complex renaming logic out of the critical timing path for subsequent executions, allowing the processor to maintain high core frequency by simply replaying pre-computed mappings from the cache rather than performing complex real-time RAT lookups.
Data Source
AI summary
Techniques for register renaming caching are described. In an embodiment, an apparatus includes a register renaming cache, front-end circuitry, lookup circuitry, and execution circuitry. The register renaming cache is to store register renaming information associated with an instruction trace. The register renaming information is to be learned from a first execution of the instruction trace and is to be used to perform register renaming in connection with a second execution of the instruction trace. The front-end circuitry is to provide, based on the instruction trace, operations for execution. The lookup circuitry to look in the register renaming cache for entries corresponding to the operations. The execution circuitry is to execute the instruction trace.