Register Rename Unit Zero Cycle Move Optimization
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Solution Overview
Problem
Current microprocessor technologies face limitations in reducing the latency and power consumption of data move operations due to the serialization caused by read-after-write dependencies and the limitations of content addressable memory (CAM) in managing duplicate register mappings.
Innovation Solution
Implementing a register rename unit that identifies zero cycle move operations, assigns the same physical register number to source and destination operands, and marks the move instruction for completion, thereby bypassing the value to younger instructions with data dependencies, while maintaining duplicate mappings in the free list to reduce latency and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If a CAM is used to maintain duplicate mappings for physical registers, then the latency of move operations is reduced, but the power consumption increases
Solution Approach 1:
The patent extracts the duplicate mapping information from a power-consuming CAM structure and stores it instead in a free list data structure that is already present in the register renaming mechanism. This eliminates the need for a separate CAM while preserving the ability to quickly identify and reuse physical registers for zero-cycle move operations, thereby reducing power consumption while maintaining latency benefits.
Solution Approach 2:
The free list, which originally served only to track available physical registers, is enhanced to also store duplicate mapping information. This multi-functional use of the free list eliminates the need for a dedicated CAM structure, reducing overall power consumption while maintaining the capability to support zero-cycle move operations.
2Productivity
If the number of entries in the CAM is increased to handle more physical register duplications, then the performance of move operations is improved, but the power consumption and circuitry complexity increase
Solution Approach 1:
The patent removes the dependency on CAM power consumption by extracting the duplicate mapping storage function from the CAM and relocating it to the free list. This allows the system to support a larger number of physical register duplications without proportionally increasing power consumption, as the free list uses less power per entry than CAM.
Solution Approach 2:
The patent uses the free list, which is a lower-cost (in terms of power and complexity) data structure compared to CAM, to store duplicate mapping information. This substitution allows for greater flexibility in the number of supported duplications without the high power cost of expanding CAM capacity.
3Productivity
If register renaming is used to allow parallel execution, then instruction level parallelism is improved, but the complexity of the register management system increases
Solution Approach 1:
The patent makes the free list serve multiple functions: tracking available physical registers and storing duplicate mapping information. This consolidation reduces the overall complexity of the register management system compared to having separate structures for each function, while still enabling the parallel execution benefits of register renaming.
Solution Approach 2:
The patent merges the duplicate mapping storage function with the existing free list structure used in register renaming. This combination simplifies the overall system architecture by eliminating redundant data structures and reducing the complexity of managing multiple separate components.
4Loss of time
If move operations are converted to zero cycle moves, then the latency is reduced, but the accuracy of dependency tracking may be compromised
Solution Approach 1:
The patent uses the free list as a feedback mechanism to track which physical registers have duplicate mappings. This allows the system to accurately identify when a move operation can be converted to a zero-cycle move while maintaining proper dependency tracking, as the free list provides real-time information about register aliasing relationships.
Solution Approach 2:
The patent pre-computes and stores duplicate mapping information in the free list before move operations occur. This preliminary action enables the system to quickly and accurately determine whether a move operation qualifies for zero-cycle conversion, maintaining dependency tracking accuracy while minimizing latency during actual execution.
Data Source
AI summary
A system and method for reducing the latency of data move operations. A register rename unit within a processor determines whether a decoded move instruction qualifies for a zero cycle move operation. If so, control logic assigns a physical register identifier associated with a source operand of the move instruction to the destination operand of the move instruction. Additionally, the register rename unit marks the given move instruction to prevent it from proceeding in the processor pipeline. Further maintenance of the particular physical register identifier may be done by the register rename unit during commit of the given move instruction.