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Add optimization section to README.

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This commit is contained in:
Stephen Chung 2020-03-12 21:19:34 +08:00
parent 7e9a4fd965
commit da440a5dff
1 changed files with 113 additions and 20 deletions
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README.md
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@ -181,21 +181,22 @@ engine.consume(
x.len() + y // returning i64
}
fn hello(x) { // script-functions can be overloaded: this one takes only one parameter
fn hello(x) { // functions can be overloaded: this one takes only one parameter
x * 2 // returning i64
}
", true)?; // pass true to 'retain_functions' otherwise these functions will be cleared
// at the end of consume()
", true)?; // pass true to 'retain_functions' otherwise these functions
// will be cleared at the end of consume()
// Evaluate the function in the AST, passing arguments into the script as a tuple if there are more than one.
// Beware, arguments must be of the correct types because Rhai does not have built-in type conversions.
// If you pass in arguments of the wrong type, the Engine will not find the function.
// Evaluate the function in the AST, passing arguments into the script as a tuple
// if there are more than one. Beware, arguments must be of the correct types because
// Rhai does not have built-in type conversions. If you pass in arguments of the wrong type,
// the Engine will not find the function.
let result: i64 = engine.call_fn("hello", &ast, ( String::from("abc"), 123_i64 ) )?;
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ put arguments in a tuple
let result: i64 = engine.call_fn("hello", 123_i64)?
// ^^^^^^^ this one calls the 'hello' function with one parameter (so need for tuple)
// ^^^^^^^ calls 'hello' with one parameter (no need for tuple)
```
Values and types
@ -255,8 +256,8 @@ Rhai's scripting engine is very lightweight. It gets its ability from the funct
```rust
use rhai::{Engine, EvalAltResult};
use rhai::RegisterFn; // include the `RegisterFn` trait to use `register_fn`
use rhai::{Dynamic, RegisterDynamicFn}; // include the `RegisterDynamicFn` trait to use `register_dynamic_fn`
use rhai::RegisterFn; // use `RegisterFn` trait for `register_fn`
use rhai::{Dynamic, RegisterDynamicFn}; // use `RegisterDynamicFn` trait for `register_dynamic_fn`
// Normal function
fn add(x: i64, y: i64) -> i64 {
@ -336,7 +337,7 @@ Your function must return `Result<_, EvalAltResult>`. `EvalAltResult` implements
```rust
use rhai::{Engine, EvalAltResult, Position};
use rhai::RegisterResultFn; // include the `RegisterResultFn` trait to use `register_result_fn`
use rhai::RegisterResultFn; // use `RegisterResultFn` trait for `register_result_fn`
// Function that may fail
fn safe_divide(x: i64, y: i64) -> Result<i64, EvalAltResult> {
@ -540,21 +541,19 @@ fn main() -> Result<(), EvalAltResult>
let mut scope = Scope::new();
// Then push some initialized variables into the state
// NOTE: Remember the default numbers used by Rhai are i64 and f64.
// Better stick to them or it gets hard to work with other variables in the script.
// NOTE: Remember the system number types in Rhai are i64 (i32 if 'only_i32') ond f64.
// Better stick to them or it gets hard working with the script.
scope.push("y".into(), 42_i64);
scope.push("z".into(), 999_i64);
// First invocation
// (the second boolean argument indicates that we don't need to retain function definitions
// because we didn't declare any!)
engine.eval_with_scope::<()>(&mut scope, false, r"
engine.eval_with_scope::<()>(&mut scope, r"
let x = 4 + 5 - y + z;
y = 1;
")?;
// Second invocation using the same state
let result = engine.eval_with_scope::<i64>(&mut scope, false, "x")?;
let result = engine.eval_with_scope::<i64>(&mut scope, "x")?;
println!("result: {}", result); // should print 966
@ -668,7 +667,8 @@ let age = 42;
let record = full_name + ": age " + age;
record == "Bob C. Davis: age 42";
// Strings can be indexed to get a character (disabled with the 'no_index' feature)
// Strings can be indexed to get a character
// (disabled with the 'no_index' feature)
let c = record[4];
c == 'C';
@ -1052,7 +1052,8 @@ debug("world!"); // prints "world!" to stdout using debug formatting
### Overriding `print` and `debug` with callback functions
```rust
// Any function or closure that takes an &str argument can be used to override print and debug
// Any function or closure that takes an &str argument can be used to override
// print and debug
engine.on_print(|x| println!("hello: {}", x));
engine.on_debug(|x| println!("DEBUG: {}", x));
@ -1063,7 +1064,7 @@ let mut log: Vec<String> = Vec::new();
engine.on_print(|s| log.push(format!("entry: {}", s)));
engine.on_debug(|s| log.push(format!("DEBUG: {}", s)));
// Evalulate script
// Evaluate script
engine.eval::<()>(script)?;
// 'log' captures all the 'print' and 'debug' output
@ -1072,6 +1073,98 @@ for entry in log {
}
```
Optimizations
=============
Rhai includes an _optimizer_ that tries to optimize a script after parsing. This can reduce resource utilization and increase execution speed.
For example, in the following:
```rust
{
let x = 999; // NOT eliminated - Rhai doesn't check yet whether a variable is used later on
123; // eliminated - no effect
"hello"; // eliminated - no effect
[1, 2, x, x*2, 5]; // eliminated - no effect
foo(42); // NOT eliminated - the function 'foo' may have side effects
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
}
```
Rhai attempts to eliminate _dead code_ (i.e. code that does nothing, for example an expression by itself as a statement, which is allowed in Rhai).
The above script optimizes to:
```rust
{
let x = 999;
foo(42);
666
}
```
Constant propagation is used to remove dead code:
```rust
if true || some_work() { print("done!"); } // since '||' short-circuits, 'some_work' is never called
if true { print("done!"); } // <-- the line above is equivalent to this
print("done!"); // <-- the line above is further simplified to this
// because the condition is always true
```
These are quite effective for template-based machine-generated scripts where certain constant values are spliced into the script text in order to turn on/off certain sections.
Beware, however, that most operators are actually function calls, and those functions can be overridden, so they are not optimized away:
```rust
if 1 == 1 { ... } // '1==1' is NOT optimized away because you can define
// your own '==' function to override the built-in default!
```
### Here be dragons!
Some optimizations can be quite aggressive and can alter subtle semantics of the script. For example:
```rust
if true { // <-- condition always true
123.456; // <-- eliminated
hello; // <-- eliminated, EVEN THOUGH the variable doesn't exist!
foo(42) // <-- promoted up-level
}
// The above optimizes to:
foo(42)
```
Nevertheless, if you would be evaluating the original script, it would have been an error - the variable `hello` doesn't exist, so the script would have been terminated at that point with an error return.
In fact, any errors inside a statement that has been eliminated will silently _go away_:
```rust
print("start!");
if my_decision { /* do nothing... */ } // <-- eliminated due to no effect
print("end!");
// The above optimizes to:
print("start!");
print("end!");
```
In the script above, if `my_decision` holds anything other than a boolean value, the script should have been terminated due to a type error.
However, after optimization, the entire `if` statement is removed, thus the script silently runs to completion without errors.
It is usually a bad idea to depend on a script failing or such kind of subtleties, but if it turns out to be necessary (why? I would never guess),
there is a setting in `Engine` to turn off optimizations.
```rust
let engine = rhai::Engine::new();
engine.set_optimization(false); // turn off the optimizer
```
[ChaiScript]: http://chaiscript.com/
[scripting languages for Rust]: https://github.com/rust-unofficial/awesome-rust#scripting
[awesome-rust]: https://github.com/rust-unofficial/awesome-rust