Struct egg::Runner

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pub struct Runner<L: Language, N: Analysis<L>, IterData = ()> {
    pub egraph: EGraph<L, N>,
    pub iterations: Vec<Iteration<IterData>>,
    pub roots: Vec<Id>,
    pub stop_reason: Option<StopReason>,
    pub hooks: Vec<Box<dyn FnMut(&mut Self) -> Result<(), String>>>,
    /* private fields */
}
Expand description

Faciliates running rewrites over an EGraph.

One use for EGraphs is as the basis of a rewriting system. Since an egraph never “forgets” state when applying a Rewrite, you can apply many rewrites many times quite efficiently. After the egraph is “full” (the rewrites can no longer find new equalities) or some other condition, the egraph compactly represents many, many equivalent expressions. At this point, the egraph is ready for extraction (see Extractor) which can pick the represented expression that’s best according to some cost function.

This technique is called equality saturation in general. However, there can be many challenges in implementing this “outer loop” of applying rewrites, mostly revolving around which rules to run and when to stop.

Runner is egg’s provided equality saturation engine that has reasonable defaults and implements many useful things like saturation checking, egraph size limits, and customizable rule scheduling. Consider using Runner before rolling your own outer loop.

Here are some of the things Runner does for you:

  • Saturation checking

    Runner checks to see if any of the rules added anything new to the EGraph. If none did, then it stops, returning StopReason::Saturated.

  • Iteration limits

    You can set a upper limit of iterations to do in case the search doesn’t stop for some other reason. If this limit is hit, it stops with StopReason::IterationLimit.

  • EGraph size limit

    You can set a upper limit on the number of enodes in the egraph. If this limit is hit, it stops with StopReason::NodeLimit.

  • Time limit

    You can set a time limit on the runner. If this limit is hit, it stops with StopReason::TimeLimit.

  • Rule scheduling

    Some rules enable themselves, blowing up the EGraph and preventing other rewrites from running as many times. To prevent this, you can provide your own RewriteScheduler to govern when to run which rules.

    BackoffScheduler is the default scheduler.

Runner generates Iterations that record some data about each iteration. You can add your own data to this by implementing the IterationData trait. Runner is generic over the IterationData that it will be in the Iterations, but by default it uses ().

Example

use egg::{*, rewrite as rw};

define_language! {
    enum SimpleLanguage {
        Num(i32),
        "+" = Add([Id; 2]),
        "*" = Mul([Id; 2]),
        Symbol(Symbol),
    }
}

let rules: &[Rewrite<SimpleLanguage, ()>] = &[
    rw!("commute-add"; "(+ ?a ?b)" => "(+ ?b ?a)"),
    rw!("commute-mul"; "(* ?a ?b)" => "(* ?b ?a)"),

    rw!("add-0"; "(+ ?a 0)" => "?a"),
    rw!("mul-0"; "(* ?a 0)" => "0"),
    rw!("mul-1"; "(* ?a 1)" => "?a"),
];

pub struct MyIterData {
    smallest_so_far: usize,
}

type MyRunner = Runner<SimpleLanguage, (), MyIterData>;

impl IterationData<SimpleLanguage, ()> for MyIterData {
    fn make(runner: &MyRunner) -> Self {
        let root = runner.roots[0];
        let mut extractor = Extractor::new(&runner.egraph, AstSize);
        MyIterData {
            smallest_so_far: extractor.find_best(root).0,
        }
    }
}

let start = "(+ 0 (* 1 foo))".parse().unwrap();
// Runner is customizable in the builder pattern style.
let runner = MyRunner::new(Default::default())
    .with_iter_limit(10)
    .with_node_limit(10_000)
    .with_expr(&start)
    .with_scheduler(SimpleScheduler)
    .run(rules);

// Now we can check our iteration data to make sure that the cost only
// got better over time
for its in runner.iterations.windows(2) {
    assert!(its[0].data.smallest_so_far >= its[1].data.smallest_so_far);
}

println!(
    "Stopped after {} iterations, reason: {:?}",
    runner.iterations.len(),
    runner.stop_reason
);

Fields

egraph: EGraph<L, N>

The EGraph used.

iterations: Vec<Iteration<IterData>>

Data accumulated over each Iteration.

roots: Vec<Id>

The roots of expressions added by the with_expr method, in insertion order.

stop_reason: Option<StopReason>

Why the Runner stopped. This will be None if it hasn’t stopped yet.

hooks: Vec<Box<dyn FnMut(&mut Self) -> Result<(), String>>>

The hooks added by the with_hook method, in insertion order.

Implementations

Create a new Runner with the given analysis and default parameters.

Sets the iteration limit. Default: 30

Sets the egraph size limit (in enodes). Default: 10,000

Sets the runner time limit. Default: 5 seconds

Add a hook to instrument or modify the behavior of a Runner. Each hook will run at the beginning of each iteration, i.e. before all the rewrites.

If your hook modifies the e-graph, make sure to call rebuild.

Example
let rules: &[Rewrite<SymbolLang, ()>] = &[
    rewrite!("commute-add"; "(+ ?a ?b)" => "(+ ?b ?a)"),
    // probably some others ...
];

Runner::<SymbolLang, ()>::default()
    .with_expr(&"(+ 5 2)".parse().unwrap())
    .with_hook(|runner| {
         println!("Egraph is this big: {}", runner.egraph.total_size());
         Ok(())
    })
    .run(rules);

Change out the RewriteScheduler used by this Runner. The default one is BackoffScheduler.

Add an expression to the egraph to be run.

The eclass id of this addition will be recorded in the roots field, ordered by insertion order.

Replace the EGraph of this Runner.

Run this Runner until it stops. After this, the field stop_reason is guaranteed to be set.

Enable explanations for this runner’s egraph. This allows the runner to explain why two expressions are equivalent with the explain_equivalence function.

By default, egg runs a greedy algorithm to reduce the size of resulting explanations (without complexity overhead). Use this function to turn this algorithm off.

By default, egg runs a greedy algorithm to reduce the size of resulting explanations (without complexity overhead). Use this function to turn this algorithm on again if you have turned it off.

Disable explanations for this runner’s egraph.

Get an explanation for why an expression matches a pattern.

Prints some information about a runners run.

Creates a Report summarizing this Runners run.

Trait Implementations

Formats the value using the given formatter. Read more

Returns the “default value” for a type. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.