Microprocessor Runahead Poison Bit Tracking
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Solution Overview
Problem
Microprocessors face inefficiencies due to speculative execution during runahead, where invalid results are generated, leading to potential cache pollution and increased processing delays, as existing methods require separate poison bits for architectural and runahead visibility, resulting in hardware inefficiencies.
Innovation Solution
The method involves using architectural poison bits to track invalid results during runahead, allowing them to serve both architectural and runahead poison functionalities on a single bit, enabling the microprocessor to make progress by altering operations and maintaining the pre-runahead state, thus reducing the need for additional bits and enhancing instruction throughput.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If separate poison bits are used for architectural and runahead visibility, then invalid results can be tracked accurately, but hardware complexity increases
Solution Approach 1:
The patent combines architectural poison bits and runahead poison bits into a single shared poison bit structure. The runahead execution unit and architectural execution unit both utilize the same poison bit array to track invalid results, eliminating the need for separate poison bit storage and reducing hardware complexity while maintaining tracking accuracy for both execution modes.
Solution Approach 2:
The poison bits are designed to serve multiple functions: they track invalid results for architectural execution, track invalid results during runahead execution, and enable both execution units to share the same validation mechanism. This multi-functional design reduces the overall hardware footprint while maintaining comprehensive invalid result tracking.
2Productivity
If speculative execution continues during runahead, then additional stalls can be uncovered, but cache pollution increases due to invalid results
Solution Approach 1:
The patent converts the potentially harmful effect of generating invalid results during runahead into a beneficial validation mechanism. By using poison bits to mark invalid results, the system can identify and prevent cache pollution from these invalid results while maintaining speculative execution. The harmful invalid data is transformed into a detectable condition that triggers prevention actions.
Solution Approach 2:
The poison bit mechanism provides feedback about the validity of results produced during runahead execution. When an invalid result is detected through the poison bit, the system can prevent that result from polluting the cache or being committed to the architectural state. This feedback loop enables continuous speculative execution while maintaining cache integrity.
3Productivity
If runahead execution is performed, then pipeline efficiency improves by uncovering stalls, but delays increase due to invalid result handling
Solution Approach 1:
The patent performs preliminary validation by setting poison bits during runahead execution before invalid results can cause delays. By proactively marking invalid results with poison bits during the runahead phase, the system prevents subsequent delays that would occur if invalid results were processed later. This preliminary action eliminates the need for corrective delays after invalid results are generated.
Data Source
AI summary
Embodiments related to managing potentially invalid results generated/obtained by a microprocessor during runahead are provided. In one example, a method for operating a microprocessor includes causing the microprocessor to enter runahead upon detection of a runahead event. The example method also includes, during runahead, determining that an operation associated with an instruction referencing a storage location would produce a potentially invalid result based on a value of an architectural poison bit associated with the storage location and performing a different operation in response.