TOC Register Prediction for Pointer Access Latency
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Solution Overview
Problem
Existing computing systems face performance degradation due to the expensive and delayed access to variables that depend on a Table of Contents (TOC) or Global Offset Table (GOT), as they require sequences of instructions to compute pointer values which are not always readily available in processors.
Innovation Solution
A computer-implemented method that replaces instruction sequences for computing TOC or GOT pointers with a Set TOC Register (STR) instruction, predicting the pointer value and using a TOC cache for high-speed access, thereby reducing the need for expensive computation and interlocks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If instruction sequences are used to compute TOC/GOT pointer values, then the pointer can be accurately calculated, but the access time increases and processing speed decreases
Solution Approach 1:
The patent pre-computes and stores TOC/GOT pointer values in a dedicated register (TOC register) before they are needed for variable access. The compiler generates code that loads the TOC pointer into a register upfront, so that subsequent variable accesses can directly use this pre-computed value without re-calculating it, thereby reducing access time while maintaining accuracy
Solution Approach 2:
The patent introduces a dedicated TOC register as an intermediary between the instruction sequence and the variable access operations. This register holds the pre-computed TOC/GOT pointer value and serves as a mediator that eliminates the need for repeated computation sequences, thus reducing access time while preserving pointer value accuracy
2Loss of information
If computed registers are used for pointer calculation, then the pointer value can be obtained, but the registers are not always readily available causing delays
Solution Approach 1:
The patent creates a dedicated TOC register that serves multiple purposes: it stores the TOC/GOT base address, provides the basis for all variable accesses in the current scope, and eliminates the need to repeatedly use general-purpose computed registers. This universal register is always readily available and eliminates delays associated with computing register availability
Solution Approach 2:
The TOC register is self-sufficient in that it directly holds the pointer value needed for variable access without requiring additional computation or dependency on other registers. The register serves itself by providing the base address that all subsequent variable accesses can directly utilize, eliminating the need to wait for computed registers to be available
3Reliability
If sequences of instructions are used to access variables depending on TOC, then the variables can be accessed, but the access process becomes expensive and delayed
Solution Approach 1:
The patent extracts the expensive computation sequence from the variable access path and separates it into a preliminary initialization phase. The TOC pointer is computed once and stored in a dedicated register, while subsequent variable accesses only require simple register operations. This extraction removes the computational burden from the critical access path, improving efficiency while maintaining correctness
Solution Approach 2:
The patent performs the expensive TOC pointer computation in advance and stores the result in a dedicated register before any variable accesses occur. This preliminary action ensures that the pointer value is ready and available when needed, eliminating delays during actual variable access operations while ensuring correctness through proper initialization
Data Source
AI summary
Table of Contents (TOC)-setting instructions are replaced in code with TOC predicting instructions. A determination is made as to whether code includes an instruction sequence to compute a value of a pointer to a reference data structure, such as a TOC. Based on determining the code includes the instruction sequence, the instruction sequence in the code is replaced with a set instruction. The set instruction predicts the value of the pointer to the reference data structure.