# 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