Predecode Unit Speculative Instruction Grouping Cache Boundaries
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Solution Overview
Problem
Conventional superscalar processors face inefficiencies in instruction grouping due to limited predecode information and lack of global context, leading to suboptimal group formation and degraded processor performance, especially when dealing with cache boundaries.
Innovation Solution
The implementation of a predecode unit that provides feedback on speculative group formation and employs a cost function to determine the effectiveness of decode-time instruction optimization (DTIO), allowing for the grouping of instructions based on compatible properties and historical performance data, and discontinuing speculative optimizations that do not improve processor performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If instructions are grouped for decode-time instruction optimization across cache boundaries, then processor performance is improved, but the complexity of instruction grouping logic increases
Solution Approach 1:
The patent segments the instruction stream into groups that align with cache boundaries. The instruction grouping logic is divided into: (1) identifying cache boundary locations, (2) forming instruction groups within each cache line, and (3) handling boundary cases separately. This segmentation allows the system to achieve performance improvements without overwhelming complexity by treating different regions of the instruction stream differently.
Solution Approach 2:
The patent performs preliminary analysis of instruction properties and cache boundary locations before actual instruction grouping and execution. The system pre-identifies which instructions can be grouped together and which cache boundaries exist, then uses this pre-computed information to guide the grouping process. This preliminary action reduces the complexity of real-time decision-making during instruction execution.
2Speed
If speculative group formation is used to optimize instruction execution, then execution speed is improved, but the accuracy of group formation decreases
Solution Approach 1:
The patent implements feedback mechanisms where the results of speculative group formation are evaluated and used to refine future grouping decisions. The system monitors whether speculative groups actually improve performance and adjusts the grouping strategy accordingly. This feedback loop allows the system to maintain high execution speed while improving the accuracy of group formation over time through learned patterns.
Solution Approach 2:
The patent changes parameters such as group size, boundary alignment, and instruction selection criteria based on performance measurements and historical data. By dynamically adjusting these parameters, the system can optimize for speed when conditions favor speculative execution while maintaining accuracy when conditions require more precise group formation. This parameter adaptation allows the system to balance speed and accuracy based on runtime conditions.
3Reliability
If instructions are fetched from main memory due to cache miss, then instruction availability is ensured, but execution latency increases
Solution Approach 1:
The patent performs preliminary actions to predict and pre-fetch instructions that are likely to be needed before actual execution. By analyzing instruction patterns and cache usage, the system proactively loads instructions into the cache before they are strictly needed, reducing the likelihood of cache misses and the associated latency. This preliminary action maintains instruction availability while minimizing time losses.
Solution Approach 2:
The patent implements mechanisms to skip or rush through cache miss penalties by overlapping memory access with other processing activities. When a cache miss is detected, the system initiates memory fetching while simultaneously performing other operations that do not depend on the missing instructions. This allows the system to maintain forward progress and reduce the effective latency impact of cache misses by utilizing otherwise idle time.
Data Source
AI summary
A technique for processing instructions includes examining instructions in an instruction stream of a processor to determine properties of the instructions. The properties indicate whether the instructions may belong in an instruction sequence subject to decode-time instruction optimization (DTIO). Whether the properties of multiple ones of the instructions are compatible for inclusion within an instruction sequence of a same group is determined. The instructions with compatible ones of the properties are grouped into a first instruction group. The instructions of the first instruction group are decoded subsequent to formation of the first instruction group. Whether the first instruction group actually includes a DTIO sequence is verified based on the decoding. Based on the verifying, DTIO is performed on the instructions of the first instruction group or is not performed on the instructions of the first instruction group.