diff --git a/src/cfg/domtree.h b/src/cfg/domtree.h
index 5753a311402..a718d4036a4 100644
--- a/src/cfg/domtree.h
+++ b/src/cfg/domtree.h
@@ -43,7 +43,8 @@ namespace wasm {
 //  etc.
 //
 // The BasicBlock type is assumed to have a ".in" property which declares a
-// vector of pointers to the incoming blocks, that is, the predecessors.
+// vector of pointers to the incoming blocks, that is, the predecessors, and a
+// ".contents.index" property to hold each block's index in `blocks`.
 template<typename BasicBlock> struct DomTree {
   std::vector<Index> iDoms;
 
@@ -83,9 +84,8 @@ DomTree<BasicBlock>::DomTree(std::vector<std::unique_ptr<BasicBlock>>& blocks) {
   }
 
   // Map basic blocks to their indices.
-  std::unordered_map<BasicBlock*, Index> blockIndices;
   for (Index i = 0; i < numBlocks; i++) {
-    blockIndices[blocks[i].get()] = i;
+    blocks[i]->contents.index = i;
   }
 
   // Initialize the iDoms array. The entry starts with its own index, which is
@@ -107,14 +107,14 @@ DomTree<BasicBlock>::DomTree(std::vector<std::unique_ptr<BasicBlock>>& blocks) {
       auto& preds = blocks[index]->in;
       Index newParent = nonsense;
       for (auto* pred : preds) {
-        auto predIndex = blockIndices[pred];
+        Index predIndex = pred->contents.index;
 
         // In a reducible graph, we only need to care about the predecessors
-        // that appear before us in the reverse postorder numbering. The only
-        // predecessor that can appear *after* us is a loop backedge, but that
+        // that appear before us in the reverse postorder numbering. Any
+        // predecessor that appears at or after us is a loop backedge, which
         // will never dominate the loop - the loop is dominated by its single
         // entry (since it is reducible, it has just one entry).
-        if (predIndex > index) {
+        if (predIndex >= index) {
           continue;
         }
 
diff --git a/src/cfg/wto.h b/src/cfg/wto.h
index b12a57ff74e..95b8101fe4d 100644
--- a/src/cfg/wto.h
+++ b/src/cfg/wto.h
@@ -115,12 +115,6 @@ WeakTopologicalOrdering<BasicBlock>::WeakTopologicalOrdering(
     return;
   }
 
-  for (Index i = 0; i < numBlocks; ++i) {
-    blocks[i]->contents.index = i;
-  }
-
-  // TODO: Avoid building an unordered_map of block indices in DomTree when
-  // BasicBlock already stores its RPO index on `contents`.
   DomTree<BasicBlock> domTree(blocks);
 
   auto isReachable = [&](Index i) {
@@ -253,8 +247,8 @@ WeakTopologicalOrdering<BasicBlock>::WeakTopologicalOrdering(
 // fixed-point analysis over its basic blocks using a Weak Topological Ordering.
 //
 // Usage:
-//   1. Construct `WTOWorklist work(cfg);` (which initializes `inQueue` and
-//      `index` on each block's `contents`).
+//   1. Construct `WTOWorklist work(cfg);` (which initializes `inQueue` on each
+//      block's `contents`).
 //   2. Seed the initial block(s) to evaluate via `work.push(cfg.entry);`.
 //   3. Call `work.run([&](BasicBlock* block) { ... });`. Inside the visitor
 //      callback, evaluate the transfer function for `block` and call
@@ -272,11 +266,8 @@ template<typename CFG> struct WTOWorklist {
   CFG& cfg;
 
   WTOWorklist(CFG& cfg) : cfg(cfg) {
-    auto& basicBlocks = cfg.basicBlocks;
-    for (Index i = 0; i < basicBlocks.size(); ++i) {
-      auto& contents = basicBlocks[i]->contents;
-      contents.inQueue = false;
-      contents.index = i;
+    for (auto& block : cfg.basicBlocks) {
+      block->contents.inQueue = false;
     }
   }
 
diff --git a/src/passes/OnceReduction.cpp b/src/passes/OnceReduction.cpp
index 814483d51e0..0075c020c79 100644
--- a/src/passes/OnceReduction.cpp
+++ b/src/passes/OnceReduction.cpp
@@ -217,6 +217,9 @@ struct Scanner : public WalkerPass<PostWalker<Scanner>> {
 
 // Information in a basic block.
 struct BlockInfo {
+  // For DomTree.
+  Index index;
+
   // We track relevant expressions, which are call to "once" functions, and
   // writes to "once" globals.
   std::vector<Expression*> exprs;
diff --git a/test/example/domtree.cpp b/test/example/domtree.cpp
index e1b93ccc437..b7ce78d18de 100644
--- a/test/example/domtree.cpp
+++ b/test/example/domtree.cpp
@@ -7,6 +7,9 @@
 using namespace wasm;
 
 struct BasicBlock {
+  struct {
+    Index index;
+  } contents;
   std::vector<BasicBlock*> in;
 
   void addPred(BasicBlock* pred) { in.push_back(pred); }