Macro-Op Cache Loop Unrolling for Higher Hit Rates
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Solution Overview
Problem
Conventional microprocessor caches face inefficiencies due to low hit rates in micro-op caches, leading to increased power consumption and performance penalties from frequent switching between instruction and micro-op caches, and fragmentation issues in variable length micro-op cache entries, along with suboptimal handling of multi-block traces.
Innovation Solution
Implementing a macro-op cache with single-fetch and multi-fetch block entries, including sequential and loop unrolled entries, to optimize cache usage by predicting consistent instruction sequences and unrolling loops, thereby reducing power consumption and improving performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a micro-op cache is used to store decoded micro-operations, then instruction execution speed is improved, but power consumption increases due to frequent cache switching and low hit rates
Solution Approach 1:
The patent segments the micro-op cache into multiple ways (e.g., 4 ways) where each way maintains independent replacement information. This segmentation allows the cache to track usage patterns for different instruction sequences separately, improving hit rates by selecting the most recently used way while reducing unnecessary cache switching and associated power consumption.
Solution Approach 2:
The patent implements dynamic replacement information tracking that adapts to different instruction patterns. The replacement information is updated based on actual cache access patterns, allowing the system to dynamically optimize for frequently accessed micro-op sequences while minimizing power consumption from cache switching.
2Adaptability or versatility
If variable length micro-op cache entries are implemented to accommodate different instruction types, then instruction set coverage is improved, but cache fragmentation increases
Solution Approach 1:
The patent applies local quality by allowing different cache entries to have different lengths based on their specific instruction content. Each cache entry is allocated exactly the space needed for its micro-operations, rather than using fixed-size entries. This reduces wasted space and fragmentation while maintaining full adaptability to various instruction types.
3Productivity
If loop unrolling is implemented in multi-fetch block entries, then processing throughput is improved, but cache entry complexity increases
Solution Approach 1:
The patent performs loop unrolling in advance by detecting loop patterns and pre-computing multiple loop iterations before they are executed. The unrolled loop instructions are stored as a single multi-fetch block cache entry, which reduces the number of cache misses and improves throughput. The complexity is managed by using systematic detection and generation algorithms that automate the unrolling process.
Data Source
AI summary
A microprocessor includes a prediction unit (PRU) that predicts a sequence of fetch blocks (FBlks) in a program instruction stream, a macro-op (MOP) cache (MOC) that comprises MOC entries (MEs), and a fusion engine. A FBlk comprises a sequential run of architectural instructions. An ME holds MOPs into which architectural instructions of one or more FBlks are decoded and includes training fields, updated by the PRU as the PRU predicts the ME in the program instruction stream. The PRU, in response to detecting that the training fields of an ME indicate the ME has been a loop body ME that has exhibited a consistent loop iteration count within the program instruction stream, requests the fusion engine to use F copies of the MOPs of the loop body ME to build in the MOC an unrolled loop multi-FBlk ME; F is at least two.