Last Branch Record Circuit Cycle Timer Logging
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Solution Overview
Problem
Current technologies face challenges in attributing microarchitectural performance issues across bottlenecks in code segments, as existing instrumentation methods are coarse-grained, intrusive, and inaccurate, making it difficult to identify low-level performance issues.
Innovation Solution
The implementation of a last branch record (LBR) event logging system that captures precise event occurrences and cycle accurate timings between branch entries, allowing for direct attribution of timed LBR latencies to performance monitoring events and identifying microarchitectural issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If traditional instrumentation methods are used to monitor performance, then device complexity is reduced, but measurement precision deteriorates resulting in coarse-grained and inaccurate performance data
Solution Approach 1:
The last branch record circuit automatically captures branch information, cycle counts, and performance monitoring event counts without requiring external instrumentation. The circuit serves itself by utilizing existing processor resources (branch logic, cycle counter, performance monitoring counters) to generate precise performance data, eliminating the need for complex external measurement devices while achieving high measurement precision
Solution Approach 2:
The last branch record circuit performs multiple functions simultaneously: it records branch target addresses, counts execution cycles, and captures performance monitoring events. This multi-functional approach consolidates what would traditionally require separate instrumentation tools into a single integrated circuit, improving measurement precision without proportionally increasing device complexity
2Loss of information
If coarse-grained instrumentation is used, then device complexity is low, but loss of information increases making it difficult to identify low-level performance issues
Solution Approach 1:
The performance monitoring system is segmented into distinct functional components: the last branch record circuit that captures branch information, the cycle counter that tracks execution cycles, and the performance monitoring counters that record specific events. This segmentation allows each component to capture specific details without requiring a complex monolithic instrumentation system, reducing information loss while keeping device complexity manageable
3Measurement precision
If precise event logging is implemented, then measurement precision improves, but loss of time increases due to additional logging overhead
Solution Approach 1:
The last branch record circuit merges the timing function with the branch recording function. The cycle counter is integrated into the same circuit that captures branch information, allowing precise timing data to be collected without requiring separate timing instrumentation. This merging eliminates redundant operations and reduces logging overhead time while maintaining high measurement precision
Data Source
AI summary
Systems, methods, and apparatuses relating to circuitry to implement precise last branch record event logging in a processor are described. In one embodiment, a hardware processor core includes an execution circuit to execute instructions, a retirement circuit to retire executed instructions, a status register, and a last branch record circuit to, in response to retirement by the retirement circuit of a first taken branch instruction, start a cycle timer and a performance monitoring event counter, and in response to retirement by the retirement circuit of a second taken branch instruction, that is a next taken branch instruction in program order after the first taken branch instruction, write values from the cycle timer and the performance monitoring event counter into a first entry in the status register and clear the values from the cycle timer and the performance monitoring event counter.