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Eagerly evaluate built-in operators for OptimizationLevel::Simple.

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Stephen Chung
2020-10-05 10:27:31 +08:00
parent b91a073596
commit 0d0affd5e9
11 changed files with 182 additions and 104 deletions
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18
doc/src/engine/optimize/eager.md
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@@ -3,8 +3,8 @@ Eager Function Evaluation When Using Full Optimization Level
{{#include ../../links.md}}
When the optimization level is [`OptimizationLevel::Full`], the [`Engine`] assumes all functions to be _pure_ and will _eagerly_
evaluated all function calls with constant arguments, using the result to replace the call.
When the optimization level is [`OptimizationLevel::Full`], the [`Engine`] assumes all functions to be _pure_
and will _eagerly_ evaluated all function calls with constant arguments, using the result to replace the call.
This also applies to all operators (which are implemented as functions).
@@ -14,8 +14,8 @@ For instance, the same example above:
// When compiling the following with OptimizationLevel::Full...
const DECISION = 1;
// this condition is now eliminated because 'DECISION == 1'
if DECISION == 1 { // is a function call to the '==' function, and it returns 'true'
// this condition is now eliminated because 'sign(DECISION) > 0'
if DECISION.sign() > 0 { // is a call to the 'sign' and '>' functions, and they return 'true'
print("hello!"); // this block is promoted to the parent level
} else {
print("boo!"); // this block is eliminated because it is never reached
@@ -24,13 +24,3 @@ if DECISION == 1 { // is a function call to the '==' function, and it r
print("hello!"); // <- the above is equivalent to this
// ('print' and 'debug' are handled specially)
```
Because of the eager evaluation of functions, many constant expressions will be evaluated and replaced by the result.
This does not happen with [`OptimizationLevel::Simple`] which doesn't assume all functions to be _pure_.
```rust
// When compiling the following with OptimizationLevel::Full...
let x = (1+2)*3-4/5%6; // <- will be replaced by 'let x = 9'
let y = (1>2) || (3<=4); // <- will be replaced by 'let y = true'
```