Macro-Op Cache Restart Abort Handling in Microprocessors
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Solution Overview
Problem
Existing microprocessor designs face inefficiencies in micro-op cache management, leading to increased power consumption and performance penalties due to mispredictions and fragmentation, particularly in high-performance superscalar processors with complex decode logic and variable-length instructions.
Innovation Solution
Implement a macro-op cache with single-fetch and multi-fetch block entries, utilizing a prediction unit to optimize cache usage by building and extending multi-fetch block entries based on program instruction stream consistency and loop unrolling, reducing the need for decode operations and improving cache hit rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional micro-op cache is used with standard replacement policies, then the cache structure is simple, but cache hit rate decreases due to fragmentation and mispredictions
Solution Approach 1:
The cache is segmented into single-fetch block entries and multi-fetch block entries, allowing different replacement policies to be applied to different segments. This segmentation enables the system to maintain high cache hit rates by treating frequently accessed multi-fetch blocks differently from regular single-fetch blocks, thereby resolving the contradiction between reliability and complexity.
Solution Approach 2:
The system performs preliminary actions by building multi-fetch block entries in advance based on prediction unit analysis of program instruction stream consistency. By proactively creating and extending multi-fetch blocks before they are needed, the system improves cache hit rates while managing complexity through structured prediction and building processes.
2Productivity
If more decode logic is added to handle variable-length instructions, then instruction processing capability improves, but power consumption increases
Solution Approach 1:
The prediction unit performs preliminary analysis of the program instruction stream to identify consistent multi-fetch blocks before decode operations are executed. By pre-processing and predicting instruction patterns, the system reduces the frequency of full decode operations, thereby maintaining high instruction processing capability while significantly reducing power consumption.
Solution Approach 2:
The macro-op cache serves itself by using the prediction unit to automatically identify and build multi-fetch block entries based on observed instruction stream patterns. This self-service mechanism reduces the need for external control and minimizes power-consuming decode operations by leveraging the cache's own usage statistics to optimize its content.
3Productivity
If the cache line size is increased to reduce fetch operations, then the number of fetch operations decreases, but cache fragmentation increases
Solution Approach 1:
The cache entries are segmented into single-fetch blocks and multi-fetch blocks with different size characteristics. Single-fetch blocks use standard cache line sizes, while multi-fetch blocks use extended sizes that span multiple cache lines. This segmentation allows the system to achieve high fetch operation efficiency through multi-fetch blocks while maintaining good cache utilization through proper management of single-fetch blocks.
Solution Approach 2:
The system dynamically changes the effective cache line size parameter based on the type of block being accessed. For multi-fetch blocks, the system uses larger effective cache line sizes that cover multiple instruction sequences, reducing fetch operations. For single-fetch blocks, standard cache line sizes are used, preventing fragmentation. This parameter change strategy resolves the contradiction between fetch efficiency and utilization efficiency.
4Loss of time
If multi-fetch block entries are built based on program consistency prediction, then decode latency is reduced, but prediction accuracy requirements increase
Solution Approach 1:
The prediction unit applies partial prediction by building multi-fetch block entries only for instruction streams that meet a threshold of consistency, rather than attempting to predict all instruction patterns. This partial action approach reduces decode latency for predictable patterns while avoiding the need for excessively high prediction accuracy across all cases, thereby resolving the contradiction between time loss and measurement precision.
Data Source
AI summary
A microprocessor includes a macro-op (MOP) cache (MOC) having MOC entries (MEs) holding MOPs. An instruction fetch circuit fetches from the MOC the MOPS of an ME for execution, and a back-end detects that a MOP of the ME is causing a need for an abort type within a predetermined subset of abort types. A control circuit uses a location within the ME of the abort-causing MOP to determine an abort point that separates the ME into a former portion and a latter portion, flushes the MOPs of the latter portion of the ME, and allows the MOPs of the former portion of the ME to execute. After the flush of the MOPs of the latter portion of the ME, the microprocessor re-fetches and executes the MOPs of the latter portion of the ME but does not re-execute the MOPs of the former portion of the ME.