updated structure of how we document the optimizations

2026-05-20 09:26:22 +02:00
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+# Optimization: Inlining
+
+- **Author**: Elliot
+- **Predecessor**: variant_baseline.cpp
+- **File(s)**: variants/variant_inlining_1.cpp ... variants/variant_inlining_7.cpp, headers/pedestrian_reduced.hpp
+- **Status**: positive (~+13% perf)
+
+## What this changes
+
+Manually inlined `w`, `acceleration_term_dt`, `pedestrian_repulsion_dt`,
+`obstacle_repulsion_dt`, `norm`, `dot`, and `find_nearest_obstacle_point` all
+into `run_simulation`. Got rid of small-number EPS checks after verifying they
+do not affect results. Removed all references to `Vector2d` and rewrote
+`Pedestrian` as a struct of doubles. Reduced `sqrt()` calls using mathematical
+properties.
+
+Built up progressively across 7 variants (`inlining_1` through `inlining_7`),
+each variant adding one more change on top of the previous one. See the file
+headers in each variant for the specific incremental change.
+
+## Code (relevant snippet)
+
+TBD - see `variants/variant_inlining_7.cpp` for the final version, and the
+individual file headers for the incremental steps.
+
+## Benchmark
+
+Conditions: single run (TODO: switch to min-of-N), N_ITER=1000,
+-O3 -march=native -flto. Predecessor reference for the same config is the
+baseline row.
+
+### x86
+
+**Machine**:
+
+```
+Processors: 16 x AMD Ryzen AI 7 350 w/ Radeon 860M
+Memory: 32 GiB of RAM (30.6 GiB usable)
+Graphics Processor: AMD Radeon 860M Graphics
+Manufacturer: LENOVO
+Product Name: 83HX
+System Version: IdeaPad Slim 5 14AKP10
+```
+
+**Frequency**: 2 GHz
+
+Existing single-point measurement (N unspecified, kept from the previous
+OPTIMIZATIONS.md; W matches roughly N=120 at N_ITER=1000):
+
+| Variant  | W(n) [Flops]     | T(n) [Cycles]  | Performance [Flops/Cycle] |
+| -------- | ---------------- | -------------- | ------------------------- |
+| Baseline | 1'058'400'000.00 | 850'352'900.00 | 1.24                      |
+| Inlining | 1'058'400'000.00 | 756'088'260.00 | 1.40                      |
+
+Inlining is about 12.9% faster on this point. A full N sweep with min-of-3 is TODO.
+
+Full sweep (TODO):
+
+| N    | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Predecessor |
+| ---- | ------------ | ------------- | ------------------------- | ------------------- |
+| 50   |              |               |                           |                     |
+| 100  |              |               |                           |                     |
+| 200  |              |               |                           |                     |
+| 500  |              |               |                           |                     |
+| 1000 |              |               |                           |                     |
+
+### ARM (Apple)
+
+**Machine**:
+
+```
+TBD
+```
+
+**Frequency**: TBD
+
+| N    | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Predecessor |
+| ---- | ------------ | ------------- | ------------------------- | ------------------- |
+| 50   |              |               |                           |                     |
+| 100  |              |               |                           |                     |
+| 200  |              |               |                           |                     |
+| 500  |              |               |                           |                     |
+| 1000 |              |               |                           |                     |
+
+## Analysis
+
+TBD - speedup likely comes from a mix of: eliminating function-call overhead at
+the ~2*N^2 hot-call sites, the struct-of-doubles `Pedestrian` improving
+register allocation, and the algebraic sqrt reductions cutting expensive ops.
+Worth confirming by looking at the generated assembly for the inner loop in
+`variant_inlining_7.cpp`.
+
+## Takeaway
+
+TBD