Predicate Logic Circuitry for VLIW Processor Pipeline Stall Reduction
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Solution Overview
Problem
In Very Long Instruction Word (VLIW) processor architectures, there is a dependency between compare and branch operations that leads to pipeline stalls, which existing technologies fail to adequately address due to limited branch prediction and increased power consumption.
Innovation Solution
A processor with multiple parallel execution pipelines that executes compare and dependent instructions concurrently, using predicate logic circuitry to provide the compare result early to the branch instruction, allowing for simultaneous execution and reducing dependency-related latencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If compare and branch instructions are executed sequentially with traditional dependency handling, then the branch instruction can correctly depend on the compare result, but pipeline stalls occur reducing processor productivity
Solution Approach 1:
The processor is divided into multiple independent execution pipelines (first execution pipeline for compare instruction, second execution pipeline for branch instruction) that can operate simultaneously. This segmentation allows the compare and branch instructions to be executed in parallel without traditional sequential dependency, resolving the contradiction between reliable dependency handling and processor throughput.
Solution Approach 2:
Predicate logic circuitry acts as an intermediary between the first execution pipeline (compare) and the second execution pipeline (branch). The predicate logic receives the compare result and selectively controls the commitment of the branch instruction result, enabling correct dependency handling while maintaining parallel execution and preventing pipeline stalls.
2Productivity
If branch prediction techniques are used to eliminate branch dependencies, then processor productivity improves, but device complexity and power consumption increase
Solution Approach 1:
The patent extracts the dependency resolution function from complex branch prediction logic and implements it instead through simple predicate logic circuitry. By taking out the need for sophisticated branch prediction hardware and replacing it with straightforward predicate-based control, the solution improves productivity while avoiding increased device complexity and power consumption.
3Productivity
If compare result is made available early to dependent instructions, then pipeline stalls are reduced improving productivity, but the risk of committing incorrect results increases
Solution Approach 1:
The predicate logic circuitry provides feedback control by monitoring the compare result and selectively enabling or disabling the commitment of the branch instruction result. This feedback mechanism ensures that even when the compare result is made available early to the second execution pipeline, only correct results are committed, preventing reliability issues while maintaining high productivity.
Data Source
AI summary
In a particular embodiment, a method is disclosed that includes receiving an instruction packet including a first instruction and a second instruction that is dependent on the first instruction at a processor having a plurality of parallel execution pipelines, including a first execution pipeline and a second execution pipeline. The method further includes executing in parallel at least a portion of the first instruction and at least a portion of the second instruction. The method also includes selectively committing a second result of executing the at least a portion of the second instruction with the second execution pipeline based on a first result related to execution of the first instruction with the first execution pipeline.