# Optimization: SoA (float version)

- **Author**: Linus
- **Predecessor**: variant_float_inlining.cpp
- **File(s)**: variants/variant_float_inlining_SoA.cpp
- **Status**: TBD

## What this changes

Same SoA layout switch as the double `variant_inlining_7_SoA`, but starting
from the float baseline instead of the double one. `Pedestrian` struct is
replaced by file-scope `vector<float>` arrays (`pos_x`, `pos_y`, `sb_e_x`,
`sb_e_y`, `sb_sq`, `e_x`, `e_y`, `w_x`, `w_y`, `curr_vel_x`, `curr_vel_y`,
`desired_v0`, `v_max`, `destination_x`, `destination_y`, `color`). Board
obstacles stay AoS.

Mostly a setup step for the float AVX2 variant. Same reasoning as the
double SoA: gets the j-side loads contiguous so the upcoming 8-wide AVX2
loop has clean aligned chunks.

The j loop also keeps the `r_ab_norm > EPS ? : EPS` and `two_b_sq > EPS_SQ
? : EPS_SQ` clamps from the float baseline. Float precision around close
encounters can collapse those to zero where double would not, and the clamp
keeps the division finite.

## Code (relevant snippet)


See `variants/variant_float_inlining_SoA.cpp`.

## Benchmark

Conditions: min of N runs (TBD), N_ITER=1000, -O3 -march=native -flto.
Diff is vs `variant_float_inlining`.

### x86

**Machine**: TBD

**Frequency**: TBD

| N    | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Predecessor |
| ---- | ------------ | ------------- | ------------------------- | ------------------- |
| 50   |              |               |                           |                     |
| 100  |              |               |                           |                     |
| 200  |              |               |                           |                     |
| 500  |              |               |                           |                     |

### ARM (Apple)

**Machine**: TBD

**Frequency**: TBD

| N    | W(n) [Flops] | T(n) [Cycles] | Performance [Flops/Cycle] | Diff vs Predecessor |
| ---- | ------------ | ------------- | ------------------------- | ------------------- |
| 50   |              |               |                           |                     |
| 100  |              |               |                           |                     |
| 200  |              |               |                           |                     |
| 500  |              |               |                           |                     |

## Analysis

Layout change only, so the expectation is roughly no win on its own. The
float scalar loop already auto-vectorises in pieces with `-O3
-march=native`, so SoA doesnt buy much over what the AoS version was
already doing. The real payoff lands in the AVX2 child.

Verify passes against `baseline_float_n60_i500.csv` (from
`variant_float_baseline`) at the 1cm tolerance.

## Takeaway

Setup step for the AVX2 variant.