Selective Instruction Fetch Stalling for Pipeline Hazard Management
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Solution Overview
Problem
Processors face pipeline hazards that hinder instruction fetch throughput, leading to reduced performance due to stalled fetch operations when a hazard occurs in the pipeline stage.
Innovation Solution
The processor implements a fetch unit with stall circuitry that selectively retains fetch operations for impacted pipeline stages while allowing unaffected stages to advance, and uses age indication circuitry to manage fetch operations and merge younger operations with stalled ones, thereby maintaining throughput.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the fetch operation is stalled due to a pipeline hazard, then the correctness of instruction processing is maintained, but the fetch throughput is reduced and power consumption increases
Solution Approach 1:
The patent segments the fetch operations into multiple independent lanes (first fetch lane, second fetch lane) that can be independently controlled. When a hazard is detected in one lane, only that specific lane is stalled while other lanes continue to fetch instructions, thereby maintaining overall fetch throughput while ensuring correctness for the affected lane.
Solution Approach 2:
The patent implements dynamic control of fetch operations by selectively enabling or disabling specific fetch lanes based on hazard conditions. The system dynamically adjusts the number of active fetch lanes from one to multiple depending on whether a hazard is present, optimizing the balance between correctness and throughput.
2Reliability
If the fetch operation is stalled due to a pipeline hazard, then the hazard is properly handled, but power consumption increases due to unnecessary cycle losses
Solution Approach 1:
By segmenting fetch operations into independent lanes, the patent allows selective stalling of only the affected lane while keeping other lanes active. This reduces unnecessary power consumption that would occur if all fetch lanes were stalled, while still maintaining proper hazard handling for the impacted lane.
Solution Approach 2:
The patent applies partial action by stalling only the minimum necessary fetch operations (those in the affected lane) rather than stalling all fetch operations. This partial stalling approach maintains hazard handling correctness while minimizing power consumption from unnecessary stalls in other lanes.
3Stability of the object's composition
If multiple fetch operations are retained due to a hazard, then program order is maintained, but read port conflicts increase
Solution Approach 1:
The patent segments fetch operations into multiple lanes that can be independently managed. When a hazard occurs, only the affected lane retains its fetch operation while other lanes continue to fetch, thereby maintaining program order within each lane while reducing the total number of retained operations and minimizing read port conflicts.
Solution Approach 2:
The system dynamically adjusts which fetch operations are retained based on hazard location. By selectively retaining operations only in the affected lane and allowing operations in other lanes to proceed, the system maintains program order where necessary while reducing overall read port conflicts.
Data Source
AI summary
At least one instruction storage coupled with a fetch unit including sets of fetch circuitry each having a same plurality of pipeline stages. The sets of fetch circuitry perform fetch operations to fetch blocks of instructions from the at least one instruction storage. Stall circuitry, in response to an indication of a hazard for a given pipeline stage of a first set of fetch circuitry, retains a fetch operation for a first block of instructions at the given pipeline stage, and zero or more fetch operations for zero or more corresponding blocks of instructions at zero or more preceding pipeline stages of the first set of fetch circuitry, until the hazard has been removed. The stall circuitry advances a fetch operation for a second block of instructions from the given pipeline stage of a second set of fetch circuitry, during an initial cycle of the one or more cycles.