Pipelined Processor Hazard Control via Thread Counters
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Solution Overview
Problem
Pipelined processors face data hazards due to high latency instructions, leading to potential misordering of instructions and processor stalling, especially in multi-threaded environments where long latency instructions can cause conflicts and incorrect data usage.
Innovation Solution
Implementing a method where each thread has associated counters that are incremented for long-latency instructions and decremented upon completion, with hazard instructions being paused if the counter is non-zero, ensuring that preceding instructions are finished before processing hazard instructions, and using a round-robin execution for a subset of threads to manage data hazards.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If long-latency instructions are processed in a pipelined processor, then instruction throughput is improved, but data hazards occur causing incorrect data usage and processor stalling
Solution Approach 1:
The patent applies preliminary action by checking the completion status of preceding instructions before executing hazard instructions. The instruction decoder examines whether required data is ready before allowing the hazard instruction to proceed, preventing data hazards before they occur. This is achieved through status bits that track instruction completion states and control logic that blocks hazard instructions until prerequisites are met.
2Reliability
If hazard instructions are paused to wait for preceding instructions, then data correctness is maintained, but processor stalling increases reducing throughput
Solution Approach 1:
The patent segments the instruction stream into different categories: hazard instructions requiring data readiness checks and non-hazard instructions that can execute immediately. By segmenting instructions based on their data dependency characteristics, the processor can selectively pause only those instructions that would cause data hazards while allowing others to proceed, minimizing overall stalling impact on throughput.
Solution Approach 2:
The patent implements dynamic control where the execution of hazard instructions is adjusted in real-time based on the completion status of preceding instructions. The instruction decoder dynamically determines whether to pause or proceed with each hazard instruction based on current pipeline state, rather than using a static approach. This dynamic adaptation allows the processor to maintain data correctness while minimizing unnecessary pauses.
3Productivity
If multiple threads are processed simultaneously, then processor utilization is improved, but data hazards between threads increase causing more conflicts
Solution Approach 1:
The patent applies universality by implementing a hazard detection mechanism that works across multiple threads using the same instruction decoder and control logic. The status bits and checking procedures are thread-agnostic, allowing the same hardware structures to handle hazard detection for any thread currently in the pipeline. This multi-functional approach enables multi-threaded processing without proportionally increasing hazard detection complexity.
Data Source
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AI summary
There is provided a method and processor for processing a thread. The thread comprises a plurality of sequential instructions, the plurality of sequential instructions comprising some short-latency instructions and some long-latency instructions and at least one hazard instruction, the hazard instruction requiring one or more preceding instructions to be processed before the hazard instruction is processed. The method comprises the steps of: a) before processing each long-latency instruction, incrementing by one, a counter associated with the thread; b) after each long-latency instruction has been processed, decrementing by one, the counter associated with the thread; c) before processing each hazard instruction, checking the value of the counter associated with the thread, and i) if the counter value is zero, processing the hazard instruction, or ii) if the counter value is non-zero, pausing processing of the hazard instruction until a later time. The processor includes means for performing steps a), b) and c) of the method.