Use RefCell to allow path compression in union-find

When I wrote the original version I didn't understand the interior
mutability pattern, but now I do! With this commit, we should have a
more optimal union-find implementation.

Author: tekknolagi

zjit/src/hir.rs

@@ -619,7 +619,7 @@ impl<T: Copy + Into<usize> + PartialEq> UnionFind<T> {
     }
 
     /// Find the set representative for `insn` without doing path compression.
-    pub fn find_const(&self, insn: T) -> T {
+    fn find_const(&self, insn: T) -> T {
         let mut result = insn;
         loop {
             match self.at(result) {
@@ -645,7 +645,7 @@ pub struct Function {
     // TODO: get method name and source location from the ISEQ
 
     insns: Vec<Insn>,
-    union_find: UnionFind<InsnId>,
+    union_find: std::cell::RefCell<UnionFind<InsnId>>,
     insn_types: Vec<Type>,
     blocks: Vec<Block>,
     entry_block: BlockId,
@@ -657,7 +657,7 @@ impl Function {
             iseq,
             insns: vec![],
             insn_types: vec![],
-            union_find: UnionFind::new(),
+            union_find: UnionFind::new().into(),
             blocks: vec![Block::default()],
             entry_block: BlockId(0),
         }
@@ -740,7 +740,7 @@ impl Function {
         macro_rules! find {
             ( $x:expr ) => {
                 {
-                    self.union_find.find_const($x)
+                    self.union_find.borrow_mut().find($x)
                 }
             };
         }
@@ -749,12 +749,12 @@ impl Function {
                 {
                     BranchEdge {
                         target: $edge.target,
-                        args: $edge.args.iter().map(|x| self.union_find.find_const(*x)).collect(),
+                        args: $edge.args.iter().map(|x| find!(*x)).collect(),
                     }
                 }
             };
         }
-        let insn_id = self.union_find.find_const(insn_id);
+        let insn_id = self.union_find.borrow_mut().find(insn_id);
         use Insn::*;
         match &self.insns[insn_id.0] {
             result@(PutSelf | Const {..} | Param {..} | NewArray {..} | GetConstantPath {..}
@@ -822,12 +822,12 @@ impl Function {
     /// Replace `insn` with the new instruction `replacement`, which will get appended to `insns`.
     fn make_equal_to(&mut self, insn: InsnId, replacement: InsnId) {
         // Don't push it to the block
-        self.union_find.make_equal_to(insn, replacement);
+        self.union_find.borrow_mut().make_equal_to(insn, replacement);
     }
 
     fn type_of(&self, insn: InsnId) -> Type {
         assert!(self.insns[insn.0].has_output());
-        self.insn_types[self.union_find.find_const(insn).0]
+        self.insn_types[self.union_find.borrow_mut().find(insn).0]
     }
 
     /// Check if the type of `insn` is a subtype of `ty`.
@@ -1963,19 +1963,19 @@ mod union_find_tests {
     }
 
     #[test]
-    fn test_find_const_returns_target() {
+    fn test_find_returns_target() {
         let mut uf = UnionFind::new();
         uf.make_equal_to(3, 4);
-        assert_eq!(uf.find_const(3usize), 4);
+        assert_eq!(uf.find(3usize), 4);
     }
 
     #[test]
-    fn test_find_const_returns_transitive_target() {
+    fn test_find_returns_transitive_target() {
         let mut uf = UnionFind::new();
         uf.make_equal_to(3, 4);
         uf.make_equal_to(4, 5);
-        assert_eq!(uf.find_const(3usize), 5);
-        assert_eq!(uf.find_const(4usize), 5);
+        assert_eq!(uf.find(3usize), 5);
+        assert_eq!(uf.find(4usize), 5);
     }
 
     #[test]