Microprocessor Macro-Op Caching with Sequential Multi-Fetch Entries
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Solution Overview
Problem
Conventional microprocessor cache systems face inefficiencies in micro-op caching due to fragmentation, conflict misses, and power consumption issues, particularly in high-performance superscalar designs, leading to increased decode latency and power consumption.
Innovation Solution
The implementation of a macro-op cache with single-fetch and multi-fetch block entries, where macro-ops are predicted and optimized, allowing for efficient caching and fusion of multiple fetch blocks, reducing the need for decode operations and minimizing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If a conventional micro-op cache is used, then the cache size is reduced, but the cache hit rate decreases due to fragmentation and conflict misses
Solution Approach 1:
The cache is divided into multiple ways (e.g., 4-way set associative), where each way maintains independent replacement information. This segmentation allows parallel tracking of usage history for multiple entries, reducing conflict misses while maintaining compact cache size.
Solution Approach 2:
The patent introduces a new dimension for cache management by adding parallel replacement information structures (such as multiple LRU stacks or counter arrays) for each cache set. This dimensional extension enables more sophisticated replacement decisions without significantly increasing physical cache size.
2Reliability
If replacement information is updated on every cache access, then the cache hit rate is improved through better replacement decisions, but the power consumption increases
Solution Approach 1:
Instead of updating replacement information on every single cache access, the system employs periodic or selective updates based on access patterns. For example, updates occur only on cache misses or at scheduled intervals, reducing the frequency of update operations and associated power consumption while maintaining effective replacement policies.
Solution Approach 2:
The patent dynamically adjusts replacement policy parameters such as the depth of LRU stacks or counter values based on observed workload characteristics. This adaptability allows the system to optimize between hit rate improvement and power consumption by modifying update frequency and complexity according to actual cache usage patterns.
3Reliability
If a larger cache line size is used, then the cache hit rate improves due to better spatial locality, but the decode latency increases
Solution Approach 1:
The cache line is segmented into multiple micro-operation fields, each representing a discrete executable unit. This segmentation allows the decode logic to process and execute micro-ops in parallel or in smaller batches, reducing the effective decode latency while maintaining the benefits of larger cache line sizes for spatial locality.
Solution Approach 2:
Instructions are pre-decoded into micro-operations during the cache fill process or in a separate decode stage before execution. This preliminary action prepares the instruction stream in advance, so that when instructions are fetched from the cache, they are already in an executable format, reducing the critical path decode latency.
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. An ME includes an expected next ME identifier and a counter, updated by the PRU as it predicts the ME in the program instruction stream, that is an indicator of consistency of sequence in the program instruction stream of the ME and an ME indicated by the expected next ME identifier. The PRU detects that each of the counters of N MEs indicate a highly consistent sequence of the N MEs and a final ME identified by the expected next ME identifier of a last ME in the sequence and instructs the fusion engine to use the MOPs of the N MEs and of the final ME to build in the MOC a sequential multi-FBlk ME.