SoA implementation

2026-05-21 19:08:46 +02:00
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+# Optimization: Struct of Arrays (SoA)
+
+- **Author**: Linus
+- **Predecessor**: variant_inlining_7.cpp
+- **File(s)**: variants/variant_inlining_7_SoA.cpp
+- **Status**: TBD
+
+## What this changes
+
+Replaced the `Pedestrian` struct with parallel `vector<double>` arrays, one
+per field (`pos_x`, `pos_y`, `e_x`, `e_y`, `sb_e_x`, `sb_e_y`, `sb_sq`, ...).
+Board obstacles are left as AoS since there are only a handful of them.
+
+Mostly a setup step for AVX2: with SoA the inner ped-ped loop reads
+contiguous chunks of `pos_x[j..]`, which is what we'll want for aligned
+vector loads.
+
+## Code (relevant snippet)
+
+```cpp
+vector<double> pos_x, pos_y;
+vector<double> sb_sq, sb_e_x, sb_e_y;
+vector<double> e_x, e_y;
+vector<double> curr_vel_x, curr_vel_y;
+// ...
+```
+
+See `variants/variant_inlining_7_SoA.cpp`.
+
+## Benchmark
+
+Conditions: 2 runs, N_ITER=1000, -O3 -march=native -flto. Diff is vs
+`variant_inlining_7`.
+On my local machine i had an improvement of about 10-15% for low N and about 5% speed up for higher N
+
+### x86
+
+**Machine**: TBD
+
+**Frequency**: TBD
+
+| 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
+
+No in depth Analysis done, just thought i wanna share it so you can also use it for further optimizations
+
+## Takeaway
+
+Layout change, not a performance win by itself. Sets up the AVX2 variant.