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​​Polonius Walkthrough #2

​​Agenda

• ​​How data is collected and packaged up in rustc

• ​​How it gets fed to polonius

• ​​What we would want to add to that for liveness

• ​​Mapping the naive polonius rules to datafrog

• ​​“how polonius works”

• ​​(Processing the results in rustc)

• ​​What it would mean to move liveness results in there

​​

​​Liveness results

​​Adding to AllFacts​ something like

​​

• ​​new parameter for variables (let’s call it V: Atom​)

• ​​variable_definitions: Vec<(V, P)>​

• ​​variable_regular_uses: Vec<(V, P)>​ “variable is regular-used at some point”

• ​​variable_drop_uses: Vec<(V, P)>​ “variable is drop-used at some point”

​​

​​Liveness computation

​​variable_regular_live(V, P) :- variable_regular_used(V, P)

​​


​​variable_regular_live(V, P) :-

​​  cfg_edge(P, Q),

​​  variable_regular_live(V, Q),

​​  !variable_defined(V, P).

​​

​​How datafrog works

​​// subset(R1, R3, P) :-

​​//   subset(R1, R2, P),

​​//   subset(R2, R3, P).

​​


​​let mut iteration = Iteration::new();

​​let subset = iteration.variable::<(Region, Region, Point)>("subset");

​​let subset_r1p = iteration.variable_indistinct("subset_r1p");

​​while iteration.changed() {

​​  subset_r1p.from_map(

​​    &subset,

​​    |&(r1, r2, p)| ((r1, p), r2));

​​subset_r2p.from_map(&subset, |&(r1, r2, p)| ((r2, p), r1));

​​


​​  subset.from_join(

​​    &subset_r2p, // ((R2, P), R1)

​​    &subset_r1p, // ((R1, P), R2)

​​    |&(_r2, p), &r1, &r3| (r1, r3, p),

​​  );

​​


​​  // subset(R1, R2, Q) :-

​​  //   subset(R1, R2, P),

​​  //   cfg_edge(P, Q),

​​  //   region_live_at(R1, Q),