> ## Documentation Index
> Fetch the complete documentation index at: https://docs.thrindex.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Compilation

> What thrindex.compile does, what the compiler resolves, and what decisions are locked at compile time.

## What compilation does

`thx.compile(model, "output.thx")` performs three things:

1. **Resolution.** Runtime hyperparameters (`tau_mem`, `dt`) are resolved to discrete constants (`alpha = exp(-dt / tau_mem)`). Once resolved, the constant is in the artifact; the hyperparameter is not.

2. **Validation.** The compiler checks the architecture against the target's capability descriptor. It rejects models that cannot be represented: missing required parameters, unsupported layer types, weight shapes inconsistent with the declared architecture.

3. **Sealing.** The artifact is written with a CRC32 integrity hash. Any post-compilation modification of the `.thx` file will cause `thrindex run` and `thrindex doctor` to report `[E0004] artifact CRC32 mismatch`.

## What is resolved at compile time

| Python parameter | Artifact field | Resolved as                    |
| ---------------- | -------------- | ------------------------------ |
| `tau_mem`        | `alpha`        | `exp(-dt / tau_mem)`           |
| `tau_syn`        | `beta`         | `exp(-dt / tau_syn)`           |
| `threshold`      | `threshold`    | passed through                 |
| `reset`          | `reset`        | `"subtract"` → 0, `"zero"` → 1 |
| `dt`             | `dt_ms`        | passed through                 |

The Python model stores `tau_mem`. The artifact stores `alpha`. You can reconstruct `tau_mem` from `alpha` and `dt_ms`, but there is no round-trip guarantee for `tau_syn` if it was not explicitly set (the default means no synaptic filter, which is distinct from `tau_syn = dt`).

## Compile targets

By default, `thx.compile` targets the behavioral simulator (`sim`). To compile for a specific hardware backend:

```python theme={null}
thx.compile(model, "model.thx", target="loihi2")
```

The compiler loads the `loihi2` capability descriptor and applies target-specific transformations:

* **Retiming**: if the target's native `dt` differs from the model's `dt`, weights and time constants are adjusted.
* **Quantization advisory**: if the target is fixed-point, the compiler reports the expected quantization error bracket based on `CONFORMANCE_ENVELOPE_V0`.
* **Delay negotiation**: if the model specifies delays and the target cannot represent them natively, the compiler inserts buffer stages or rejects.

## Compile-time advisories

The compiler prints advisories — not errors — for decisions that succeed but require attention:

```
advisory: retiming dt=1ms → dt=2ms (alpha 0.9512 → 0.9048)
advisory: delay synapse[0] d=3 exceeds hardware max d=2; emulated with buffer stage
```

Advisories are informational. The compilation succeeds. They appear in `thrindex compile --verbose`.

## What is not locked at compile time

* **Input size** (T and n\_features) is locked in the artifact. You cannot feed a compiled 100×700 model a 50×700 input without recompiling.
* **Batch size** is not locked — the artifact runs on any batch.
* **Seed** is not locked — `thrindex run --seed N` controls the RNG at inference time.

## The format version

Every artifact carries a format version (`m3` as of THRINDEX 0.2). The version controls the deserializer: a `m3` artifact is rejected by an engine that only supports `m2`. The CLI prints the format version in the run header.

Format versions are additive: `m3` adds fields that `m2` did not have, but any valid `m2` artifact can be represented as a `m3` artifact by filling new fields with defaults.

See [Concepts: Artifact Format](/concepts/artifact-format) for the complete field layout.