Floating-Point Dependency Detection Unit for Evil Twin Conditions
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Solution Overview
Problem
Computer processors face performance penalties due to 'evil twin' dependencies, where double-precision floating-point instructions depend on single-precision instructions that have not yet retired, leading to increased execution time and latency.
Innovation Solution
Inserting additional instructions into the instruction stream to facilitate earlier execution of 'evil twin' consumer instructions by storing information about execution dependencies and using a detection unit to identify and manage these dependencies, allowing values to be stored in a physical register before they are needed by subsequent instructions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the processor waits for single-precision instructions to retire before executing dependent double-precision instructions, then execution accuracy is maintained, but execution time increases and performance decreases
Solution Approach 1:
The patent applies preliminary action by detecting potential evil twin dependencies before they cause execution delays. The detection unit identifies when a double-precision instruction depends on single-precision instructions that haven't retired yet, and the system proactively inserts barrier instructions or modifies the instruction stream to prevent the dependency from blocking execution, thereby maintaining accuracy while improving performance
Solution Approach 2:
The patent uses an intermediary approach by introducing a detection unit and dependency management mechanism that acts as a mediator between the single-precision and double-precision instruction streams. This intermediary detects evil twin conditions and inserts barrier instructions or modifies execution ordering to resolve dependencies without requiring strict sequential execution, thus maintaining correctness while reducing performance penalties
2Reliability
If barrier instructions are inserted to resolve evil twin dependencies, then execution correctness is maintained, but instruction stream complexity and processing overhead increase
Solution Approach 1:
The patent applies the taking out principle by extracting the dependency detection and resolution logic from the main execution path. The detection unit operates independently to identify evil twin conditions, and barrier instructions are inserted only when necessary rather than universally. This selective approach maintains correctness while minimizing the addition of complexity to the instruction stream
Data Source
AI summary
Techniques for handling dependency conditions, including evil twin conditions, are disclosed herein. An instruction may designate a source register comprising two portions. The source register may be a double-precision register and its two portions may be single-precision portions, each specified as destinations by two other single-precision instructions. Execution of these two single-precision instructions, especially on a register renaming machine, may result in the appropriate values for the two portions of the source register being stored in different physical locations, which can complicate execution of an instruction stream. In response to detecting a potential dependency, one or more instructions may be inserted in an instruction stream to enable the appropriate values to be stored within one physical double precision register, eliminating an actual or potential evil twin dependency. Embodiments including a compiler that inserts instructions in a generated instruction stream to eliminate dependency conditions are also contemplated.