Macro-Op Cache Successor Fetch Block Prediction for Decode Latency
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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 processors with variable-length micro-ops and complex decode logic, leading to increased latency and power consumption.
Innovation Solution
A prediction unit predicts the program instruction stream to build single-fetch and multi-fetch block macro-op cache entries, optimizing micro-op caching by recognizing consistent sequences and loops, and employing a fusion engine to fuse decoded macro-ops, reducing the need for decode operations and improving cache hit rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the cache size is increased to improve cache hit rate, then the cache hit rate is improved, but the area occupied by the cache increases
Solution Approach 1:
The patent segments the micro-op cache into multiple sets, where each set contains multiple ways. This segmentation allows the cache to be organized efficiently in terms of both area and hit rate. By dividing the cache into sets and ways, the system can implement set-associative caching that improves hit rate without requiring a single large monolithic cache structure.
Solution Approach 2:
The patent implements a nested structure where ways are nested within sets, creating a hierarchical cache organization. Each set contains multiple ways, and the replacement policy operates at the way level within each set. This nested arrangement allows efficient utilization of cache area while maintaining high hit rates through multi-level access patterns.
2Reliability
If the cache line size is increased to improve cache hit rate, then the cache hit rate is improved, but the area occupied by each cache entry increases
Solution Approach 1:
The patent implements dynamic cache line sizing where the effective cache line size varies based on the micro-op fetch pattern. When sequential micro-ops are fetched, the cache utilizes the full cache line; when non-sequential access occurs, the effective size reduces. This dynamic behavior allows the cache to adapt to different access patterns, improving hit rate without permanently allocating excessive space for each entry.
3Device complexity
If conventional replacement policy is used to manage cache entries, then the replacement mechanism is simple, but conflict misses increase due to fragmentation
Solution Approach 1:
The patent implements a replacement policy that uses feedback from micro-op fetch patterns to make replacement decisions. The system monitors access patterns and uses this information to determine which ways to replace within each set. This feedback-driven approach allows the cache to adapt to actual usage patterns, reducing conflict misses while maintaining manageable complexity through localized decision-making at each set level.
Solution Approach 2:
The patent changes the replacement parameter from global LRU (least recently used) to local LRU at the way level within each set. This parameter change allows independent replacement decisions in each way, reducing the impact of fragmentation and conflict misses. By changing the granularity of the replacement parameter from cache-wide to set-local, the system reduces conflict misses without significantly increasing overall complexity.
4Measurement precision
If decode logic is made more complex to handle variable-length micro-ops, then the decode accuracy is improved, but the power consumption increases
Solution Approach 1:
The patent implements preliminary action by pre-decoding micro-ops and storing them in the micro-op cache before they are needed for execution. The decode logic operates in advance during the fetch phase, and the decoded micro-ops are cached for later use. This preliminary decoding allows the main execution phase to use the cached decoded operations, reducing the overall power consumption of the decode logic while maintaining high decode accuracy.
Solution Approach 2:
The patent uses copying by creating a copy of the decoded micro-ops in the micro-op cache. Instead of repeatedly executing the complex decode logic for the same micro-op sequences, the system copies the decoded results into the cache and reuses them. This copying approach significantly reduces power consumption by avoiding redundant decode operations while maintaining full decode accuracy for the copied micro-ops.
5Speed
If micro-op caching is implemented to reduce fetch time, then the fetch speed is improved, but the device complexity increases
Solution Approach 1:
The patent segments the micro-op cache into multiple sets with multiple ways each, allowing parallel access paths. This segmentation enables the fetch unit to access different sets simultaneously, improving fetch speed through parallelism. The segmented structure also localizes complexity within each set, making the overall system more manageable despite the increased number of components.
Solution Approach 2:
The patent implements multi-functionality by designing the micro-op cache to serve multiple purposes: caching decoded micro-ops, providing parallel access paths, and implementing flexible replacement policies. The same cache structure supports both sequential and non-sequential micro-op fetching, and the replacement mechanism handles both cache hits and misses efficiently. This universal design improves fetch speed while avoiding the need for separate specialized structures for each function.
Data Source
AI summary
A microprocessor includes a prediction unit (PRU) comprising a fetch block (FBlk) predictor (FBP) that predicts a sequence of FBlks, each FBlk having a corresponding fetch block start address (FBSA), and branch predictors; a macro-op (MOP) cache (MOC) includes MOC entries (MEs) including multi-FBlk MOC entries (MF-MEs) for holding MOPs decoded from instructions of multiple FBlks. The PRU detects a hit of a current FBSA on an MF-ME; performs a set of actions K times: looking up the current FBSA in the FBP and branch predictors to obtain outputs, using the outputs to predict a successor FBSA of a successor FBlk; and making the current FBSA the successor FBSA; and predicts that an FBSA of a successor FBlk to the MF-ME is the current FBSA resulting from performing K times the set of actions. K is a number of FBlks built into the MF-ME (alternatively times a loop unroll factor).