Macro-Op Cache Entry Fusion Across Multiple Fetch Blocks
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Solution Overview
Problem
Conventional microprocessor cache systems face inefficiencies in micro-op caching due to fragmentation, power consumption, and misprediction penalties, particularly in high-performance superscalar designs, leading to increased power consumption and decode latency.
Innovation Solution
A two-stage process is employed to build multi-fetch block macro-op cache entries, including single-fetch block and loop unrolled multi-fetch block entries, optimizing macro-ops through fusion and prediction of instruction sequences, reducing the need for decode operations and improving cache hit rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional single-stage micro-op cache system is used, then the structure is simple, but cache hit rates are reduced due to fragmentation and the system consumes more power with increased decode latency
Solution Approach 1:
The patent divides the micro-op cache system into two distinct stages: a first micro-op cache and a second micro-op cache. The first cache handles frequently accessed micro-ops while the second cache stores less frequently accessed micro-ops, reducing fragmentation in the first cache and improving overall hit rates. This segmentation allows each cache stage to be optimized for its specific access patterns without compromising the other.
Solution Approach 2:
The system performs preliminary decoding of architectural instructions into micro-ops and stores them in the first micro-op cache before execution. By pre-computing and caching micro-ops in advance, the system reduces decode latency during actual execution, as the decoded micro-ops are readily available in the cache rather than requiring real-time decoding.
2Reliability
If a larger cache size is implemented to improve hit rate, then more micro-ops can be stored, but power consumption increases
Solution Approach 1:
The patent segments the cache into two smaller stages rather than one large cache. The first micro-op cache stores frequently accessed micro-ops, while the second cache stores less frequently accessed ones. This segmentation allows the system to achieve high hit rates for common operations without maintaining a single large, power-consuming cache, as the first cache can be kept small and fast.
Solution Approach 2:
The system applies different cache retention policies and replacement strategies to different stages. The first micro-op cache uses a policy optimized for high-frequency access patterns, while the second cache uses a policy suitable for less frequent access. This local optimization allows each cache stage to be tuned for its specific access characteristics, improving overall efficiency without requiring excessive cache size.
3Loss of time
If a single-stage cache is used, then the system is simpler to implement, but decode latency increases due to fragmentation
Solution Approach 1:
The patent divides the cache into two stages to eliminate fragmentation issues in a single cache. The first micro-op cache stores micro-ops from a subset of architectural instructions, while the second cache stores micro-ops from the remaining instructions. This segmentation ensures that the first cache remains compact and efficient, reducing decode latency for the most commonly executed instructions without requiring a single large, fragmented cache.
Solution Approach 2:
The system performs preliminary decoding and caching of micro-ops in the first stage before execution needs them. By pre-decoding architectural instructions into micro-ops and storing them in the first micro-op cache, the system eliminates the need for real-time decoding during execution, significantly reducing decode latency. The two-stage structure allows this preliminary action to be efficiently organized without excessive complexity.
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), a decode unit, and a fusion engine. Each ME indicates whether it is a single-FBlk ME (SF-ME) that holds MOPs associated with a single FBlk whose architectural instructions have been decoded into the MOPs of the SF-ME or a multi-FBlk ME (ME-ME) that holds MOPs associated with multiple FBlks whose architectural instructions have been decoded into the MOPs of the MF-ME. For each FBlk of one or more FBlks in the program instruction stream: the decode unit decodes the architectural instructions of the FBlk into MOPs, and the fusion engine builds a SF-ME in the MOC using the decoded MOPs, and the fusion engine builds a MF-ME in the MOC using the MOPs of a series of SF-MEs.