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