Processor Load-Store Unit Speculative Scheduling
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Solution Overview
Problem
Modern microprocessors face inefficiencies in scheduling speculative load instructions due to deep pipelining, which leads to increased latency in receiving and using load instruction result data, particularly in scenarios like linked list traversals, where the shorter latency cases are not fully utilized.
Innovation Solution
A processor system with a scheduler that predicts whether result data for a load instruction resides in the data cache rather than the store queue, allowing younger dependent load instructions to be issued early, reducing load-to-load latency by forwarding result data directly from the cache within the load-store unit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If deep pipelining is used to support higher clock frequencies, then clock frequency and microarchitectural complexity are improved, but latency to receive and use load instruction result data increases
Solution Approach 1:
The scheduler speculatively issues the consuming load instruction before the producing load instruction completes, performing the action in advance based on prediction that the data will be available from cache. This allows the pipeline to maintain higher utilization without waiting for actual data availability.
Solution Approach 2:
The system uses feedback from the data cache status and load instruction dependencies to dynamically adjust scheduling decisions. The scheduler monitors whether result data resides in cache or store queue and adjusts issue timing accordingly, creating a closed-loop control system.
2Productivity
If data forwarding is performed through the store queue, then load results can be forwarded to dependent instructions, but appreciable delay is incurred
Solution Approach 1:
The invention extracts the data forwarding path from the traditional store queue route and creates a direct forwarding path from the data cache to the consuming load instruction. This bypasses the store queue delay and enables faster data transfer for load-to-load dependencies.
Solution Approach 2:
The scheduler acts as an intermediary that directly connects the producing load instruction's result data to the consuming load instruction when both instructions are in the pipeline, eliminating the need for store queue mediation and reducing forwarding latency.
3Reliability
If younger dependent load instructions are issued after the producing load instruction completes, then data accuracy is ensured, but load-to-load latency increases and performance is reduced
Solution Approach 1:
The consuming load instruction is issued in advance before the producing load instruction completes, based on speculative prediction that the data will be available. This preliminary action is validated by subsequent feedback to ensure data accuracy is maintained.
Solution Approach 2:
The scheduling system dynamically adjusts the issue timing of dependent instructions based on real-time conditions such as cache status and dependency resolution. The system can switch between conservative (wait for completion) and aggressive (speculative early issue) scheduling modes.
Data Source
AI summary
A system and method for efficient scheduling of dependent load instructions. A processor includes both an execution core and a scheduler that issues instructions to the execution core. The execution core includes a load-store unit (LSU). The scheduler determines a first condition is satisfied, wherein the first condition comprises result data for a first load instruction is predicted eligible for LSU-internal forwarding. The scheduler determines a second condition is satisfied, wherein the second condition comprises a second load instruction younger in program order than the first load instruction is dependent on the first load instruction. In response to each of the first condition and the second condition being satisfied, the scheduler can issue the second load instruction earlier than it otherwise would. The LSU internally forwards the received result data from the first load instruction to address generation logic for the second load instruction.