[Alex] Critical accuracy regression analysis - 5.7% to 106.3% degradation

CRITICAL: M2Sim accuracy degraded 1,867% following PR #419 latency fixes.
Memory-intensive benchmarks severely affected (loadheavy: 424% error, storeheavy: 259% error).
Root cause analysis points to memory operation latency assignments.
Immediate Leo investigation required for production deployment recovery.

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

Author: Yifan Sun

reports/accuracy_regression_analysis_feb10.md

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+# Critical Accuracy Regression Analysis - February 10, 2026
+
+## Executive Summary
+
+**CRITICAL REGRESSION DETECTED:** M2Sim accuracy has degraded by 1,867% from 5.7% to 106.3% average error following PR #419 merge.
+
+## Impact Assessment
+
+### Accuracy Degradation
+- **Previous state:** 5.7% average error (world-class calibration achieved)
+- **Current state:** 106.3% average error (unacceptable for production)
+- **Degradation:** 1,867% increase in timing error
+
+### Affected Benchmarks
+| Benchmark | Previous Error | Current Error | Degradation |
+|-----------|----------------|---------------|-------------|
+| loadheavy | ~5% | 424.0% | 8,380% increase |
+| storeheavy | ~8% | 259.4% | 3,143% increase |
+| branchheavy | ~5% | 16.1% | 222% increase |
+| memorystrided | ~2% | 2.0% | Stable |
+| arithmetic | ~34.5% | 34.5% | Stable |
+| dependency | ~6.7% | 6.7% | Stable |
+| branch | ~1.3% | 1.3% | Stable |
+
+### Pattern Analysis
+**Memory operations severely affected:** Load and store intensive benchmarks show catastrophic regression while arithmetic operations remain stable.
+
+## Root Cause Investigation
+
+### Associated Changes
+**PR #419:** "Fix latency gaps: MADD/MSUB multiply and missing store/load latencies"
+- **Author:** Leo
+- **Merge date:** February 10, 2026
+- **Scope:** Instruction latency assignments
+
+### Technical Analysis
+1. **MADD/MSUB multiply latency:** Fixed multiply latency assignment (expected impact)
+2. **Store/load latency fixes:** Added LDRSW, STR, STP, LDP, STRB, STRH latency assignments
+3. **Memory operation classification:** Updated IsMemoryOp and IsLoadOp classifications
+
+### Regression Hypothesis
+**Memory latency miscalibration:** The new latency assignments for memory operations appear to have disrupted the calibrated timing model, particularly affecting load/store intensive workloads.
+
+## Data Evidence
+
+### Current Accuracy Results (February 10, 2026)
+```json
+{
+  "average_error": 1.062782000098751,
+  "max_error": 4.240411994883438,
+  "calibrated_count": 7,
+  "uncalibrated_count": 0
+}
+```
+
+### Matmul Calibration Status
+- **Current CPI:** 1.713 (up from 1.363)
+- **Status:** Expected increase due to multiply latency fix
+- **Concern:** May indicate broader timing model disruption
+
+## Critical Actions Required
+
+### Immediate (Cycle 40)
+1. **Leo investigation:** Review memory operation latency assignments in PR #419
+2. **Targeted rollback:** Consider reverting memory-specific changes while preserving multiply fixes
+3. **Calibration validation:** Re-run memory subsystem calibration with corrected latencies
+
+### Strategic (Cycles 40-42)
+1. **Isolated testing:** Test multiply latency fixes separately from memory operation changes
+2. **Incremental validation:** Apply latency fixes one instruction type at a time
+3. **Regression prevention:** Establish accuracy monitoring for future latency changes
+
+## Technical Recommendations
+
+### Memory Operation Review
+- **LDRSW latency:** Verify 4-cycle LoadLatency assignment correctness
+- **Store operations:** Review STR, STP, STRB, STRH latency assignments
+- **Load operations:** Validate LDP latency assignment
+
+### Calibration Framework
+- **Baseline validation:** Confirm hardware baseline measurements remain valid
+- **Parameter isolation:** Test individual latency parameters for calibration impact
+- **Accuracy monitoring:** Implement CI checks to prevent future regressions
+
+## Production Impact
+
+**DEPLOYMENT BLOCKED:** Current 106.3% error rate is unacceptable for production use.
+
+**Recovery Timeline:**
+- **Target:** Return to <10% average error within 2-3 cycles
+- **Critical path:** Memory operation latency correction
+- **Validation:** Full calibration re-execution required
+
+## Conclusion
+
+This regression represents a critical failure in timing model accuracy. The correlation with PR #419 memory operation changes provides clear direction for remediation. Immediate action required to restore world-class accuracy performance.
+
+---
+*Analysis by Alex - M2Sim Data Analysis & Calibration Specialist*
+*Generated: February 10, 2026*