Process current bucket instead of parent's bucket when starting loop for dominators.

compiler/rustc_data_structures/src/graph/dominators/mod.rs

@@ -109,28 +109,27 @@ pub fn dominators<G: ControlFlowGraph>(graph: G) -> Dominators<G::Node> {
         // they have been placed in the bucket.
         //
         // We compute a partial set of immediate dominators here.
-        let z = parent[w];
-        for &v in bucket[z].iter() {
+        for &v in bucket[w].iter() {
             // This uses the result of Lemma 5 from section 2 from the original
             // 1979 paper, to compute either the immediate or relative dominator
             // for a given vertex v.
             //
             // eval returns a vertex y, for which semi[y] is minimum among
-            // vertices semi[v] +> y *> v. Note that semi[v] = z as we're in the
-            // z bucket.
+            // vertices semi[v] +> y *> v. Note that semi[v] = w as we're in the
+            // w bucket.
             //
             // Given such a vertex y, semi[y] <= semi[v] and idom[y] = idom[v].
             // If semi[y] = semi[v], though, idom[v] = semi[v].
             //
             // Using this, we can either set idom[v] to be:
-            //  * semi[v] (i.e. z), if semi[y] is z
+            //  * semi[v] (i.e. w), if semi[y] is w
             //  * idom[y], otherwise
             //
             // We don't directly set to idom[y] though as it's not necessarily
             // known yet. The second preorder traversal will cleanup by updating
             // the idom for any that were missed in this pass.
             let y = eval(&mut parent, lastlinked, &semi, &mut label, v);
-            idom[v] = if semi[y] < z { y } else { z };
+            idom[v] = if semi[y] < w { y } else { w };
         }
 
         // This loop computes the semi[w] for w.
@@ -213,10 +212,11 @@ pub fn dominators<G: ControlFlowGraph>(graph: G) -> Dominators<G::Node> {
         // If we don't yet know the idom directly, then push this vertex into
         // our semidominator's bucket, where it will get processed at a later
         // stage to compute its immediate dominator.
-        if parent[w] != semi[w] {
+        let z = parent[w];
+        if z != semi[w] {
             bucket[semi[w]].push(w);
         } else {
-            idom[w] = parent[w];
+            idom[w] = z;
         }
 
         // Optimization: We share the parent array between processed and not