Commit Frontier for Exception Handling in Block-Based Processors
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Solution Overview
Problem
Current processor architectures face challenges in efficiently handling exceptions and maintaining performance due to complex circuit resources and overhead in register renaming, dataflow analysis, and misspeculation recovery, which limits energy efficiency and performance improvements.
Innovation Solution
The implementation of a block-based processor with an Explicit Data Graph Execution (EDGE) ISA that allows individual instructions within an instruction block to commit after execution, reducing the need for large load/store queues and enabling efficient handling of exceptions by resuming execution from the interrupted point, thereby avoiding redundant operations and improving debugging capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If block-based processor executes instructions in arbitrary order based on dependency availability, then latency due to delay in receiving instruction operands is reduced, but maintaining architectural state integrity and handling exceptions becomes more complex
Solution Approach 1:
The patent segments the instruction block into individually commitable instructions with separate commit status tracking. Each instruction can be committed independently when its dependencies are satisfied, allowing out-of-order execution while maintaining a clear segmentation of committed vs. uncommitted instructions for exception handling.
Solution Approach 2:
The patent introduces a commit frontier as an intermediary mechanism that tracks the boundary between committed and uncommitted instructions. This mediator structure simplifies exception handling by providing a clear reference point for determining which instructions to rollback when an exception occurs, reducing the complexity of maintaining state integrity.
2Device complexity
If individual instructions commit after execution, then large load/store queues and other structures storing state can be reduced, but handling exceptions requires complex rollback mechanisms
Solution Approach 1:
The patent performs preliminary actions by maintaining a commit frontier that tracks which instructions have committed before an exception occurs. This advance tracking mechanism is prepared in normal execution, so when an exception happens, the rollback can quickly identify which instructions need to be undone without complex analysis, reducing rollback time.
Solution Approach 2:
The patent implements a mechanism where uncommitted instructions are automatically discarded (quashed) when an exception occurs, while committed instructions are preserved. This selective discarding and recovering approach minimizes rollback time by clearly distinguishing between instructions that can be safely discarded and those that must be recovered.
3Use of energy by moving object
If instructions execute when dependencies are available rather than in sequential order, then energy efficiency improves, but ensuring correct commit order and handling side effects becomes more difficult
Solution Approach 1:
The patent implements feedback mechanisms through dependency tracking and commit frontier monitoring. The system continuously monitors which instructions have their dependencies satisfied and updates the commit frontier accordingly, providing feedback control that ensures instructions are committed in the correct order relative to architectural state changes while allowing out-of-order execution for energy efficiency.
Solution Approach 2:
The patent introduces dynamic adjustment of commit timing based on dependency satisfaction rather than fixed sequential ordering. The commit frontier dynamically advances as instructions complete their dependencies, allowing the system to adaptively determine commit order based on actual data availability, improving energy efficiency while maintaining reliability through the dynamic tracking mechanism.
4Productivity
If large blocks of instructions are used in block-based processor, then productivity increases, but handling exceptions requires avoiding redundant memory accesses and unwanted side effects
Solution Approach 1:
The patent applies partial action by allowing only the necessary portion of the instruction block to commit before an exception occurs, rather than requiring the entire block to complete. Instructions beyond the commit frontier are quashed without executing their full operation, preventing redundant memory accesses and unwanted side effects while maintaining high productivity through partial block commitment.
Data Source
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AI summary
Systems and methods are disclosed for executing instructions with a block-based processor. Instructions can be executed in any order as their dependencies arrive, but the individual instructions are committed in a serial fashion. Further, exception handling can be performed by storing transient state for an instruction block and resuming by restoring the transient state. This allows programmers to see intermediate state for the instruction block before the subject block has committed. In one examples of the disclosed technology, a method of operating a processor executing a block-based instruction set architecture includes executing at least one instruction encoded for an instruction block, responsive to determining that an individual instruction of the instruction block can commit, advancing a commit frontier for the instruction block to include all instructions in the instruction block that can commit, and committing one or more instructions inside the advanced commit frontier.