Variable Width Unaligned Fetch Circuitry for Processor Performance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current processor architectures are limited by the need to align fetch operations to single cache line boundaries, which restricts fetch width and can lead to inefficient data retrieval, especially when approaching page boundaries, resulting in reduced performance and increased speculation without significant improvements in cache miss detection speed.
Innovation Solution
Implementing variable width unaligned fetch circuitry that adjusts fetch width to avoid crossing page boundaries within a single fetch operation, allowing for a maximum width of up to 128 bytes while reducing width when necessary, without requiring additional hardware for multiple page lookups.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If fetch operations are aligned to single cache line boundaries, then hardware complexity is reduced, but fetch width is restricted and performance decreases
Solution Approach 1:
The fetch width is made dynamic and adjustable rather than fixed. The system can adaptively select between different fetch widths (e.g., 32 bytes, 64 bytes, 128 bytes) based on the alignment of the fetch address with cache line boundaries and page boundaries, allowing optimal performance without requiring complex additional hardware for multiple page lookups
Solution Approach 2:
The fetch width parameter is changed based on the alignment conditions. When the fetch address is properly aligned, a larger fetch width (e.g., 128 bytes) is used to improve performance. When misaligned or near page boundaries, the fetch width is reduced (e.g., to 32 or 64 bytes) to avoid crossing boundaries, eliminating the need for complex hardware handling
2Speed
If fetch width is increased to improve performance, then cache miss detection speed improves, but the likelihood of crossing page boundaries increases
Solution Approach 1:
The fetch width dynamically adapts based on the position relative to page boundaries. The system calculates the distance to the next page boundary and adjusts the fetch width accordingly, allowing maximum width when safe and reduced width when approaching boundaries, thus maintaining both speed and boundary compliance
Solution Approach 2:
The system performs preliminary checks on fetch address alignment and distance to page boundaries before executing the fetch operation. This preliminary anti-action prevents boundary violations by adjusting fetch width in advance, avoiding the need for complex runtime handling of boundary crossings
3Productivity
If variable width unaligned fetch is implemented, then fetch efficiency improves, but device complexity increases
Solution Approach 1:
Different parts of the fetch circuitry handle different aspects locally: alignment detection logic identifies boundary conditions, fetch width selection logic chooses appropriate widths, and the fetch execution unit performs the actual operation. This local specialization achieves variable width fetch efficiency without requiring globally complex redesign
Solution Approach 2:
The fetch circuitry is designed to handle multiple functions using the same basic infrastructure: aligned fetch, unaligned fetch, variable width selection, and boundary avoidance all use the same core fetch mechanism, reducing overall complexity compared to implementing separate dedicated circuits for each function
Data Source
AI summary
Techniques for implementing a variable width unaligned fetch for instructions are described. In certain examples, a hardware processor core includes fetch circuitry to perform a single fetch operation to fetch from a paged memory: (i) a multiple cache line width of instruction data, between a minimum width that is greater than one cache line and a maximum width that is a plurality of cache lines, when the multiple cache line width of the instruction data does not include a page boundary of the paged memory, and (ii) less than or equal to one cache line width of the instruction data when the multiple cache line width of the instruction data does include the page boundary of the paged memory; decoder circuitry to decode a single instruction, comprising an opcode, from the instruction data into a decoded instruction; and execution circuitry to execute the decoded instruction according to the opcode.