Optimization Levels
==================

{{#include ../../links.md}}

There are three levels of optimization: `None`, `Simple` and `Full`.

* `None` is obvious - no optimization on the AST is performed.

* `Simple` (default) performs only relatively _safe_ optimizations without causing side-effects
  (i.e. it only relies on static analysis and [built-in operators] for constant [standard types],
  and will not perform any external function calls).

  However, it is important to bear in mind that _constants propagation_ is performed with the
  caveat that, if [constants] are modified later on (yes, it is possible, via Rust functions),
  the modified values will not show up in the optimized script.  Only the initialization values
  of [constants] are ever retained.

  Furthermore, overriding a [built-in operator][built-in operators] in the [`Engine`] afterwards
  has no effect after the optimizer replaces an expression with its calculated value.

* `Full` is _much_ more aggressive, _including_ calling external functions on constant arguments to determine their result.
  One benefit to this is that many more optimization opportunities arise, especially with regards to comparison operators.


Set Optimization Level
---------------------

An [`Engine`]'s optimization level is set via a call to `Engine::set_optimization_level`:

```rust
// Turn on aggressive optimizations
engine.set_optimization_level(rhai::OptimizationLevel::Full);
```