Physical Register Swap Memory Renaming for Processor Latency
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Solution Overview
Problem
Conventional processor-based devices face inefficiencies due to data dependencies, particularly true dependencies between instructions, which are not adequately addressed by register renaming and memory renaming mechanisms, leading to latency issues and increased power consumption.
Innovation Solution
The implementation of physical register (PR) swap memory renaming, where a PR swap table stores store and load dependency IDs to enable speculative bypass of store and load instructions, reducing latency and power overhead by allowing earlier execution of dependent instructions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional memory renaming mechanisms are used to track store-load dependencies, then dependency tracking capability is improved, but processor area and power consumption increase
Solution Approach 1:
The patent extracts the memory renaming functionality from the conventional front-end pipeline and relocates it to the back-end execution stage. Specifically, the PR swap table is implemented in the execution stage rather than the decode or rename stages, allowing dependency tracking to occur only when actually needed for store-load pairs, thereby reducing continuous power consumption while maintaining tracking capability.
Solution Approach 2:
The patent performs preliminary allocation of physical register tags to store and load instructions during the decode stage, but actual dependency tracking and swap table population occurs later in the execution stage when the store instruction is issued. This preliminary tagging enables fast identification without requiring continuous monitoring, reducing power while preserving reliability.
2Reliability
If conventional memory renaming mechanisms are implemented in the front end of the instruction pipeline, then dependency prediction is improved, but timing challenges and pipeline complexity increase
Solution Approach 1:
The patent segments the memory renaming functionality into distinct components: physical register tag allocation in the decode stage, store instruction issuance tracking in the execution stage, and PR swap table management separated from the main pipeline. This segmentation allows each component to operate independently with optimized timing, reducing pipeline complexity while maintaining prediction accuracy.
Solution Approach 2:
The patent introduces the PR swap table as an intermediary data structure between the store and load instructions. Rather than requiring complex front-end prediction logic, the PR swap table acts as a mediator that automatically tracks store-load dependencies using simple tag comparisons, reducing pipeline complexity while preserving dependency prediction capability.
3Reliability
If load replay processes are used to handle memory dependencies, then correctness is maintained, but latency increases
Solution Approach 1:
The patent performs preliminary population of the PR swap table with store instruction tags before the load instruction needs to access the data. When a load instruction is issued, the PR swap table already contains the mapping information, allowing immediate tag comparison and direct data forwarding without requiring load replay. This preliminary action maintains correctness while eliminating the time penalty of replay.
Solution Approach 2:
The patent enables the load instruction to skip the conventional load replay process by using the PR swap table to directly identify and forward data from the store instruction. When a match is found in the PR swap table, the load instruction rushes through to completion using the pre-established tag mapping, maintaining correctness while significantly reducing the time normally spent in replay sequences.
Data Source
AI summary
Providing physical register (PR) swap memory renaming in processor-based devices is disclosed herein. In some exemplary aspects, a processor provides an instruction processing circuit comprising a scheduling stage circuit and an execution stage circuit. The scheduling stage circuit comprises a reservation station circuit, while the execution stage circuit comprises a PR swap table storing a plurality of PR swap table entries. The scheduling stage circuit issues a first instruction that is associated with a store dependency ID. The execution stage circuit, in response to the issuing of the first instruction, identifies a PR swap table entry among the plurality of PR swap table entries corresponding to the store dependency ID, retrieves a load dependency ID of the PR swap table entry, and broadcasts the load dependency ID to the reservation station circuit to wake a second instruction that is associated with the load dependency ID.