Operation Cache for Fixed-Width Instruction Bypass
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Solution Overview
Problem
Variable-width instructions in processors incur extra hardware complexity, leading to increased power consumption and latency due to the need for additional pipeline stages for decoding, which limits instruction bandwidth and performance.
Innovation Solution
Implementing an operation cache (OC) that stores previously decoded instructions in a fixed-width format, allowing for bypassing traditional instruction cache fetch and decode stages, reducing redirect latency, and enabling wider parallel instruction dispatch.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If variable-width instructions are used, then instruction encoding flexibility is improved, but hardware complexity and power consumption increase due to extra decode stages
Solution Approach 1:
The patent pre-decodes instructions into fixed-width format and stores them in the operation cache before execution. This preliminary decoding action eliminates the need for complex variable-width decoding during normal operation, as the cache contains ready-to-execute fixed-width instructions that can be retrieved without additional decoding complexity.
Solution Approach 2:
The patent creates a copy of the original variable-width instructions in a fixed-width format within the operation cache. This copy serves as a simplified representation that preserves the essential execution information while eliminating the variable-width encoding complexity, allowing the processor to work with the simpler format during execution.
2Productivity
If variable-width instructions are decoded in real-time, then instruction bandwidth is maintained, but latency increases due to extra pipeline stages
Solution Approach 1:
Instructions are decoded in advance and stored in the operation cache in fixed-width format. When instructions are needed for execution, they can be retrieved directly from the cache without undergoing the time-consuming variable-width decoding process, thereby reducing latency while maintaining instruction bandwidth.
Solution Approach 2:
The operation cache acts as an intermediary between the instruction fetch unit and the execution units. It stores pre-decoded fixed-width instructions, mediating the data flow and eliminating the need for real-time variable-width decoding, thus reducing latency in the instruction pipeline.
3Reliability
If traditional instruction cache fetch and decode pipeline stages are used, then instruction processing is complete, but power consumption increases due to idle pipeline stages on redirects
Solution Approach 1:
The patent extracts the decoding function from the main execution path by pre-decoding instructions and storing them in the operation cache. This separation allows the processor to bypass the complex decode stages during normal operation and on redirects, retrieving fixed-width instructions directly from the cache, thereby reducing power consumption while maintaining complete instruction processing.
Solution Approach 2:
By pre-decoding instructions and storing them in the operation cache, the system prepares instructions in advance in a format ready for immediate execution. This eliminates the need to activate decode pipeline stages during redirects or normal operation, reducing power consumption while ensuring complete and accurate instruction processing when needed.
4Productivity
If variable-width instructions are decoded with high bandwidth, then more instructions per cycle are dispatched, but device complexity increases for parsing and decoding
Solution Approach 1:
Instructions are decoded in advance into fixed-width format and stored in the operation cache. This preliminary decoding enables high instructions-per-cycle dispatch because the fixed-width instructions can be retrieved and dispatched in parallel without requiring complex real-time parsing and decoding hardware, thus maintaining productivity while reducing device complexity.
Data Source
AI summary
A system and method for using an operation (op) cache is disclosed. The system and method include an op cache for caching previously decoded instructions. The op cache includes a plurality of physically indexed and tagged instructions allowing sharing of instructions between threads. The op cache is chained through multiple ways allowing service of a plurality of instructions in a cache line. The op cache is stored between a shared operation storage and immediate/displacement storage to maximize capacity.