Branch Misprediction Cost Tracking in Branch Prediction Units
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Solution Overview
Problem
Conventional performance monitoring units (PMU) in processors do not accurately report the performance cost of branch mispredictions, making it difficult to optimize software for eliminating flaky branches and complicating performance analysis in high-performance processors.
Innovation Solution
Implementing a branch prediction unit (BPU) with circuitry to track and report the performance cost of individual branch mispredictions, including the time lost due to retirement pushout cycles, and extend precise profiling hardware to provide instruction-granularity cost and misprediction type differentiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional performance monitoring units are used, then the processor can operate with standard branch prediction, but accurate measurement of branch misprediction cost is not achieved
Solution Approach 1:
The patent embeds a dedicated misprediction cost tracker within the branch prediction unit, which itself is integrated into the performance monitoring unit. This nested structure allows the cost tracking functionality to be contained within existing hierarchical processor components, achieving precise measurement without proportionally increasing overall system complexity
Solution Approach 2:
The patent introduces an intermediary misprediction cost tracker that acts as a mediator between the branch prediction unit and the performance monitoring unit. This intermediary component captures misprediction events and quantifies their cost, bridging the gap between branch prediction operations and performance measurement without requiring complete redesign of either subsystem
2Productivity
If speculative execution is used to avoid pipeline stalls, then processor performance improves, but branch mispredictions occur more frequently causing performance degradation
Solution Approach 1:
The patent implements a feedback mechanism where the misprediction cost tracker continuously monitors and quantifies the performance impact of branch mispredictions. This feedback information can be used by the branch prediction unit to adjust its prediction strategies, allowing the system to optimize the balance between speculative execution benefits and misprediction penalties
Solution Approach 2:
The patent enables dynamic adjustment of branch prediction behavior based on real-time misprediction cost measurements. The system can adaptively modify prediction thresholds, speculation depth, or recovery strategies based on the observed misprediction costs, allowing optimal performance across varying workload characteristics
3Loss of information
If detailed branch misprediction tracking is implemented, then performance bottleneck identification improves, but hardware resources are consumed
Solution Approach 1:
The patent extracts only the essential misprediction cost information from complex performance data using dedicated counter registers that track specific metrics (misprediction count, recovery cycles, performance impact). This extraction approach provides sufficient information for bottleneck identification without capturing unnecessary details that would consume excessive hardware resources
Solution Approach 2:
The patent uses parameter changes in the form of configurable tracking depth and granularity levels. The misprediction cost tracker can adjust the detail level of information collected based on performance needs, allowing the system to optimize between information quality and resource consumption by changing tracking parameters rather than hardware configuration
Data Source
AI summary
An embodiment of an integrated circuit may comprise a branch prediction unit to predict branches for an instruction decoder and circuitry coupled to the branch prediction unit, the circuitry to track a performance metric for an individual branch misprediction. Other embodiments are disclosed and claimed.