[llvm] [DomTree] Avoid duplicate hash lookups in runDFS() (NFCI) (PR #96460)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: Nikita Popov (nikic)

<details>
<summary>Changes</summary>

runDFS() currently performs three hash table lookups. One in the main loop, one when checking whether a successor has already been visited and another when adding parent and reverse children to the successor.

We can avoid the two additional lookups by making the parent number part of the stack, and then making the parent / reverse children update part of the main loop.

The main loop already has a check for already visited nodes, so we don't have to check this in advance -- we can simply push the node to the worklist and skip it later.

This results in a minor compile-time improvement: http://llvm-compile-time-tracker.com/compare.php?from=c43d5f540fd43409e7997c9fec97a1d415855b7c&to=1844fc47329a2905bf73b7b86964e8a31fa3b758&stat=instructions:u

---
Full diff: https://github.com/llvm/llvm-project/pull/96460.diff


1 Files Affected:

- (modified) llvm/include/llvm/Support/GenericDomTreeConstruction.h (+5-16) 


``````````diff
diff --git a/llvm/include/llvm/Support/GenericDomTreeConstruction.h b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
index 401cc4eb0ec1b..57cbe993d8739 100644
--- a/llvm/include/llvm/Support/GenericDomTreeConstruction.h
+++ b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
@@ -180,15 +180,17 @@ struct SemiNCAInfo {
                   unsigned AttachToNum,
                   const NodeOrderMap *SuccOrder = nullptr) {
     assert(V);
-    SmallVector<NodePtr, 64> WorkList = {V};
+    SmallVector<std::pair<NodePtr, unsigned>, 64> WorkList = {{V, AttachToNum}};
     NodeToInfo[V].Parent = AttachToNum;
 
     while (!WorkList.empty()) {
-      const NodePtr BB = WorkList.pop_back_val();
+      const auto [BB, ParentNum] = WorkList.pop_back_val();
       auto &BBInfo = NodeToInfo[BB];
+      BBInfo.ReverseChildren.push_back(ParentNum);
 
       // Visited nodes always have positive DFS numbers.
       if (BBInfo.DFSNum != 0) continue;
+      BBInfo.Parent = ParentNum;
       BBInfo.DFSNum = BBInfo.Semi = BBInfo.Label = ++LastNum;
       NumToNode.push_back(BB);
 
@@ -201,22 +203,9 @@ struct SemiNCAInfo {
             });
 
       for (const NodePtr Succ : Successors) {
-        const auto SIT = NodeToInfo.find(Succ);
-        // Don't visit nodes more than once but remember to collect
-        // ReverseChildren.
-        if (SIT != NodeToInfo.end() && SIT->second.DFSNum != 0) {
-          if (Succ != BB) SIT->second.ReverseChildren.push_back(LastNum);
-          continue;
-        }
-
         if (!Condition(BB, Succ)) continue;
 
-        // It's fine to add Succ to the map, because we know that it will be
-        // visited later.
-        auto &SuccInfo = NodeToInfo[Succ];
-        WorkList.push_back(Succ);
-        SuccInfo.Parent = LastNum;
-        SuccInfo.ReverseChildren.push_back(LastNum);
+        WorkList.push_back({Succ, LastNum});
       }
     }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/96460