# 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.