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_archive/rhai_engine/rhaibook/engine/optimize/dead-code.md

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+Dead Code Elimination
+=====================
+
+{{#include ../../links.md}}
+
+```admonish question.side.wide "But who writes dead code?"
+
+Nobody deliberately writes scripts with dead code (we hope).
+
+They are, however, extremely common in template-based machine-generated scripts.
+```
+
+Rhai attempts to eliminate _dead code_.
+
+"Dead code" is code that does nothing and has no side effects.
+
+Example is an pure expression by itself as a statement (allowed in Rhai).
+The result of the expression is calculated then immediately discarded and not used.
+
+```rust
+{
+    let x = 999;            // NOT eliminated: variable may be used later on (perhaps even an 'eval')
+    
+    123;                    // eliminated: no effect
+    
+    "hello";                // eliminated: no effect
+    
+    [1, 2, x, 4];           // eliminated: no effect
+    
+    if 42 > 0 {             // '42 > 0' is replaced by 'true' and the first branch promoted
+        foo(42);            // promoted, NOT eliminated: the function 'foo' may have side-effects
+    } else {
+        bar(x);             // eliminated: branch is never reached
+    }
+    
+    let z = x;              // eliminated: local variable, no side-effects, and only pure afterwards
+    
+    666                     // NOT eliminated: this is the return value of the block,
+                            // and the block is the last one so this is the return value of the whole script
+}
+```
+
+The above script optimizes to:
+
+```rust
+{
+    let x = 999;
+    foo(42);
+    666
+}
+```