Opcode Length Caching for Variable-Length Instruction Pipelining
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Variable-length instruction formats in computer systems lead to performance degradation in pipelining due to conditional reading operations, affecting both virtualized and non-virtualized systems.
Innovation Solution
A method and system for caching variable-length instructions, where a predictive length is determined and used to avoid conditional reads by caching the instruction length alongside the instruction, allowing for single-stage prefix reading and improved pipelining.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If variable-length instruction formats are used, then instruction density and code size are improved, but pipelining performance deteriorates due to conditional reading operations
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing the opcode length in a length cache before instruction execution. The system determines the opcode length in advance using a multi-stage decoding process, stores it in a cache structure indexed by instruction pointer, and retrieves it during execution to avoid conditional reading. This preliminary determination of opcode length enables fixed-length reads subsequent to the initial fetch, thereby maintaining high instruction density while eliminating pipelining degradation.
2Measurement precision
If two-part instruction reading is performed (prefix then rest), then variable-length instructions are accurately decoded, but system performance deteriorates due to inability to pipeline
Solution Approach 1:
The patent applies segmentation by dividing the instruction decoding process into distinct stages: prefix reading to determine opcode length, rest of instruction reading, and interpretation. By segmenting the operation and caching the length determination result separately from the instruction execution, the system maintains accurate decoding of variable-length instructions while enabling pipelining of subsequent operations. The cache stores the decoded length information that can be retrieved without blocking the instruction pipeline.
Solution Approach 2:
The patent introduces an intermediary mechanism - the opcode length cache - that mediates between the variable-length instruction format and the fixed-length read requirement for pipelining. The cache acts as an intermediary storage structure that holds the decoded opcode length, allowing the instruction fetch unit to read instructions with a fixed length determined by the cache rather than performing conditional reads based on prefix analysis during execution.
3Reliability
If conditional reading based on prefix value is used, then correct instruction length is determined, but pipelining efficiency deteriorates
Solution Approach 1:
The patent applies copying by creating a cached copy of the opcode length information in a separate data structure indexed by instruction pointer. Instead of performing conditional reading based on prefix value during each instruction execution, the system copies the length determination result into the cache during a preliminary decoding phase, then retrieves this copied length information during execution. This copying mechanism ensures reliable instruction length determination while enabling efficient pipelined execution.
Data Source
AI summary
A computer system caches variable-length instructions in a data structure. The computer system locates a first copy of an instruction in the cached data structure using a current value of the instruction pointer as a key. The computer system determines a predictive length of the instruction, and reads a portion of the instruction from an instruction memory as a second copy. The second copy has the predictive length. Based on the comparison of the first copy with the second copy, the computer system determines whether or not to read the rest of the instruction from the instruction memory, and then interprets the instruction for use by the computer system.