Control Flow Prediction Using Pointer-Indexed History Storage
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Solution Overview
Problem
In data processing systems with speculative execution, accurately predicting the behavior of control flow instructions is challenging, especially in out-of-order execution scenarios where mispredictions lead to rewinding, causing loss of previous execution results and making it difficult to determine loop iterations, which complicates replay prediction.
Innovation Solution
A data processing apparatus with history storage circuitry for sets of helper instructions' behaviors, pointer storage for indexing, increment circuitry for updating pointers based on events, and prediction circuitry to determine control flow instruction behavior, maintaining previous execution results during flushes and resetting pointers to enable accurate replay predictions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If speculative execution is used to improve performance, then instruction execution speed is improved, but prediction accuracy deteriorates due to mispredictions in out-of-order execution scenarios
Solution Approach 1:
The history storage is segmented into multiple sets, with each set dedicated to storing behaviors of helper instructions for a specific control flow instruction. This segmentation allows the system to maintain separate prediction histories for different control flow contexts, improving prediction accuracy by preventing cross-contamination of prediction data while enabling parallel speculative execution for multiple control flow paths.
Solution Approach 2:
The patent introduces a new dimension to branch prediction by storing not just the outcome of control flow instructions but also the behaviors of helper instructions that execute alongside them. This multi-dimensional history storage (control flow outcome + helper instruction behaviors) provides richer context for predictions, improving accuracy in speculative execution scenarios where traditional single-dimension prediction fails.
2Measurement precision
If history storage is maintained during flushes to preserve execution results, then prediction accuracy is improved, but device complexity increases due to additional circuitry for maintaining and managing history
Solution Approach 1:
The patent extracts the history storage function into a dedicated history storage circuitry component that is separate from the main execution pipeline. This extraction allows the history to be maintained independently during flushes without complicating the core execution logic. The history storage circuitry can be selectively preserved or cleared based on flush type, managing complexity through functional separation.
Solution Approach 2:
The history storage circuitry serves multiple functions: it stores control flow outcomes, stores helper instruction behaviors, provides prediction data, and maintains state across flushes when beneficial. This multi-functionality justifies the added complexity by consolidating what would otherwise require separate mechanisms into a single versatile component.
3Measurement precision
If multiple sets of behaviors are stored for different control flow instructions, then prediction accuracy for specific instructions is improved, but loss of information increases due to difficulty in determining which history to use
Solution Approach 1:
The patent implements feedback mechanisms where the actual outcome of control flow instructions is compared with predictions, and the history storage is updated accordingly. This feedback loop ensures that the most relevant history is maintained and that the system learns from past predictions, reducing information loss by dynamically adjusting which history entries are most valuable for future predictions.
Solution Approach 2:
The system performs preliminary actions by pre-storing helper instruction behaviors alongside control flow outcomes before predictions are needed. This preliminary storage of contextual information ensures that when a control flow instruction is encountered, the relevant history is already prepared and organized, eliminating the need for complex real-time analysis and reducing information loss.
4Measurement precision
If pointers are incremented to index behaviors correctly in loop iterations, then replay prediction accuracy is improved, but device complexity increases due to increment circuitry and pointer management
Solution Approach 1:
The pointer increment circuitry is designed to automatically increment pointers based on detected loop iteration patterns without requiring complex external control logic. The system self-services by detecting loop structures and autonomously managing pointer increments, reducing the overall complexity burden despite adding the increment function. This self-managing approach justifies the added circuitry through automation of what would otherwise require complex control logic.
Data Source
AI summary
There is provided a data processing apparatus comprising history storage circuitry that stores sets of behaviours of helper instructions for a control flow instruction. Pointer storage circuitry stores pointers, each associated with one of the sets. The behaviours in the one of the sets are indexed according to one of the pointers associated with that one of the sets. Increment circuitry increments at least some of the pointers in response to an increment event and prediction circuitry determines a predicted behaviour of the control flow instruction using one of the sets of behaviours.