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_archive/rhai_engine/rhaibook/engine/optimize/index.md

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+Script Optimization
+===================
+
+{{#title Script Optimization}}
+
+{{#include ../../links.md}}
+
+Rhai includes an _optimizer_ that tries to optimize a script after parsing.
+This can reduce resource utilization and increase execution speed.
+
+Script optimization can be turned off via the [`no_optimize`] feature.
+
+
+Optimization Levels
+===================
+
+{{#include ../../links.md}}
+
+There are three levels of optimization: `None`, `Simple` and `Full`.
+The default is `Simple`.
+
+An [`Engine`]'s optimization level is set via [`Engine::set_optimization_level`][options].
+
+```rust
+// Turn on aggressive optimizations
+engine.set_optimization_level(rhai::OptimizationLevel::Full);
+```
+
+`None`
+------
+
+`None` is obvious – no optimization on the AST is performed.
+
+
+`Simple` (Default)
+------------------
+
+`Simple` 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).
+
+```admonish warning.small
+After _constants propagation_ is performed, if the [constants] are then modified (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.
+```
+
+```admonish warning.small
+
+Overriding a [built-in operator] in the [`Engine`] afterwards has no effect after the
+optimizer replaces an expression with its calculated value.
+```
+
+`Full`
+------
+
+`Full` is _much_ more aggressive, _including_ calling external functions on [constant] arguments to
+determine their results.
+
+One benefit to this is that many more optimization opportunities arise, especially with regards to
+comparison operators.