Add LP extractor

[mwillsey]

This adds an Extractor that can do common subexpression elimination using linear programming. WIP

[remysucre]

It can be worth trying the cycle breaking encoding in https://arxiv.org/pdf/1012.1802.pdf (grep for "There is an additional subtlety in the above encoding"). It introduces n^2 variables where n is the number of enodes, but most of these variables will be inactive during ILP solving. It is complete, but will be slower than cycle-breaking a.la. tensat

[Bastacyclop]

Hi! Do you know when this feature will be merged? Is it usable as-is, if I use this branch as my dependency? I'd like to play around with LP extraction.

[mwillsey]

@Bastacyclop Yes this branch is usable and should be fully functional. I will try to merge it before the next release, and your feedback on using it would be very valuable!

[Bastacyclop]

Hi @mwillsey. I started using this branch and there might be a useful feature missing: an easy way to add an "expression" with multiple roots back into an e-graph (a DFG really).

let (best, bestRoots) = LpExtractor::new(&runner.egraph, AstSize).solve_multiple(&roots[..]);                  
{ // used to print the outcome
  let mut g: EGraph<SymbolLang, ()> = Default::default();                                    
  g.add_expr(&best); // this function does not work, it would be great to have `add_dfg`                                                       
  g.dot().to_svg("/tmp/final.svg").unwrap();                                                 
}

edit: this is what I implemented, not handling explanations:

fn add_dfg<L: Language, N: Analysis<L>>(dfg: &RecExpr<L>, egraph: &mut EGraph<L, N>) -> Vec<Id> {
    let nodes = dfg.as_ref();
    let mut eclasses = Vec::with_capacity(nodes.len());
    let mut is_root = (0..nodes.len()).map(|_| true).collect::<Vec<_>>(); // could use bit vector
    for node in nodes {
        eclasses.push(egraph.add(node.clone().map_children(|i| {
            is_root[usize::from(i)] = false;
            eclasses[usize::from(i)]
        })));
    }
    eclasses.iter().cloned().zip(is_root.iter().cloned())
        .filter_map(|(e, is_r)| if is_r { Some(e) } else { None }).collect::<Vec<_>>()
}

[Bastacyclop]

Also, I am getting a crash on:

egg/src/lp_extract.rs

Line 186 in 269c786

let v = &self.vars[&id];

When extracting from the following graph with 3 roots:

Any idea why?

[Bastacyclop]

Looks like your 'Canonicalize ids' commit was reverted somehow?

edit: canonicalizing the root ids given to the extraction function does fix the issue, but maybe this should be handled under the hood?

[mwillsey]

@Bastacyclop great catch, I think I just fixed it.