Predicting Table of Contents Pointer Values for Subroutine Branches
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Solution Overview
Problem
In computing environments, accessing variables through a Global Offset Table (GOT) or Table of Contents (TOC) can be delayed due to the reliance on computed registers that are not always readily available, leading to performance issues.
Innovation Solution
A computer program product that predicts the value of a pointer to a reference data structure before executing the sequence of instructions to compute it, using a Set TOC Register instruction and a TOC cache to facilitate early access, with mechanisms for recovery from mispredictions and verification of predicted values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sequence of instructions is used to compute the TOC pointer value, then the pointer value can be accurately obtained, but the access time is delayed due to dependency on computed registers
Solution Approach 1:
The patent predicts the TOC pointer value in advance before the actual computation is completed. The prediction mechanism estimates the pointer value based on available information, allowing the system to proceed with operations that would otherwise be blocked by the computation dependency, thereby reducing access time while maintaining accuracy through subsequent verification
Solution Approach 2:
The patent introduces a prediction mechanism as an intermediary between the instruction sequence and the TOC pointer computation. This intermediary provides estimated pointer values that enable earlier access to variables, bridging the gap between the need for immediate access and the delayed availability of computed register values
2Loss of time
If the TOC pointer value is predicted before computation, then access time is reduced, but the risk of misprediction increases
Solution Approach 1:
The patent implements a feedback mechanism where the actual computed TOC pointer value is compared with the predicted value. When a misprediction is detected, the system recovers by using the correctly computed value, ensuring reliability while maintaining the performance benefits of prediction for correct cases
Solution Approach 2:
The patent prepares recovery mechanisms in advance for potential mispredictions. By pre-establishing correction paths and alternative computation routes, the system cushions against the negative effects of misprediction, allowing it to recover quickly without significant performance penalty
3Measurement precision
If computed registers are used to calculate the TOC pointer, then the pointer value is dynamically accurate, but the registers are not always readily available
Solution Approach 1:
The patent uses inexpensive prediction mechanisms that do not rely on expensive or unavailable computed registers. The prediction can be performed using readily available information, providing a quick and efficient alternative that sacrifices some computational rigor for speed, with correction applied only when necessary
Data Source
AI summary
Predicting a Table of Contents (TOC) pointer value responsive to branching to a subroutine. A subroutine is called from a calling module executing on a processor. Based on calling the subroutine, a value of a pointer to a reference data structure, such as a TOC, is predicted. The predicting is performed prior to executing a sequence of one or more instructions in the subroutine to compute the value. The value that is predicted is used to access the reference data structure to obtain a variable value for a variable of the subroutine.