[llvm] [DomTreeConstr][NFC] Resize NodeInfos once on construction (PR #211998)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke created https://github.com/llvm/llvm-project/pull/211998

This avoids frequent size checks when accessing node infos. All numbered
graphs support getMaxNumber().

Also move away from BB->getParent() as preliminary work for numbering
VPlan blocks -- there, getParent() has a different meaning.


>From d8faf7f608142cbd518deb4c2fd2120879b5411b Mon Sep 17 00:00:00 2001
From: Alexis Engelke <engelke at in.tum.de>
Date: Sat, 25 Jul 2026 08:38:29 +0000
Subject: [PATCH] [spr] initial version

Created using spr 1.3.8-wip
---
 .../llvm/Support/GenericDomTreeConstruction.h | 35 ++++++++++---------
 llvm/unittests/Support/GenericDomTreeTest.cpp |  5 ---
 2 files changed, 18 insertions(+), 22 deletions(-)

diff --git a/llvm/include/llvm/Support/GenericDomTreeConstruction.h b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
index eb2e1e4ec0d74..05504571aa246 100644
--- a/llvm/include/llvm/Support/GenericDomTreeConstruction.h
+++ b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
@@ -107,11 +107,20 @@ template <typename DomTreeT> struct SemiNCAInfo {
   using BatchUpdatePtr = BatchUpdateInfo *;
 
   // If BUI is a nullptr, then there's no batch update in progress.
-  SemiNCAInfo(BatchUpdatePtr BUI) : BatchUpdates(BUI) {}
+  SemiNCAInfo(const DomTreeT &DT, BatchUpdatePtr BUI) : BatchUpdates(BUI) {
+    if constexpr (GraphHasNodeNumbers<NodePtr>) {
+      unsigned MaxNodeNumber =
+          GraphTraits<typename DomTreeT::ParentPtr>::getMaxNumber(DT.Parent);
+      NodeInfos.resize(MaxNodeNumber + 1); // nullptr block is zero.
+    }
+  }
 
   void clear() {
     NumToNode.clear();
-    NodeInfos.clear();
+    if constexpr (GraphHasNodeNumbers<NodePtr>)
+      NodeInfos.assign(NodeInfos.size(), InfoRec{});
+    else
+      NodeInfos.clear();
     ReverseChildren.clear();
     // Don't reset the pointer to BatchUpdateInfo here -- if there's an update
     // in progress, we need this information to continue it.
@@ -136,14 +145,6 @@ template <typename DomTreeT> struct SemiNCAInfo {
   InfoRec &getNodeInfo(NodePtr BB) {
     if constexpr (GraphHasNodeNumbers<NodePtr>) {
       unsigned Idx = BB ? GraphTraits<NodePtr>::getNumber(BB) + 1 : 0;
-      if (Idx >= NodeInfos.size()) {
-        unsigned Max = 0;
-        if (BB)
-          Max = GraphTraits<decltype(BB->getParent())>::getMaxNumber(
-              BB->getParent());
-        // Max might be zero, graphs might not support getMaxNumber().
-        NodeInfos.resize(Max ? Max + 1 : Idx + 1);
-      }
       return NodeInfos[Idx];
     } else {
       return NodeInfos[BB];
@@ -383,7 +384,7 @@ template <typename DomTreeT> struct SemiNCAInfo {
       return Roots;
     }
 
-    SemiNCAInfo SNCA(BUI);
+    SemiNCAInfo SNCA(DT, BUI);
 
     // PostDominatorTree always has a virtual root.
     SNCA.addVirtualRoot();
@@ -534,7 +535,7 @@ template <typename DomTreeT> struct SemiNCAInfo {
     assert(IsPostDom && "This function is for postdominators only");
     LLVM_DEBUG(dbgs() << "Removing redundant roots\n");
 
-    SemiNCAInfo SNCA(BUI);
+    SemiNCAInfo SNCA(DT, BUI);
 
     for (unsigned i = 0; i < Roots.size(); ++i) {
       auto &Root = Roots[i];
@@ -596,7 +597,7 @@ template <typename DomTreeT> struct SemiNCAInfo {
     }
     // This is rebuilding the whole tree, not incrementally, but PostViewBUI is
     // used in case the caller needs a DT update with a CFGView.
-    SemiNCAInfo SNCA(PostViewBUI);
+    SemiNCAInfo SNCA(DT, PostViewBUI);
 
     // Step #0: Number blocks in depth-first order and initialize variables used
     // in later stages of the algorithm.
@@ -932,7 +933,7 @@ template <typename DomTreeT> struct SemiNCAInfo {
       return false;
     };
 
-    SemiNCAInfo SNCA(BUI);
+    SemiNCAInfo SNCA(DT, BUI);
     SNCA.runDFS(Root, 0, UnreachableDescender, 0);
     SNCA.runSemiNCA();
     SNCA.attachNewSubtree(DT, Incoming);
@@ -1027,7 +1028,7 @@ template <typename DomTreeT> struct SemiNCAInfo {
     LLVM_DEBUG(dbgs() << "\tTop of subtree: " << BlockNamePrinter(ToIDomTN)
                       << "\n");
 
-    SemiNCAInfo SNCA(BUI);
+    SemiNCAInfo SNCA(DT, BUI);
     SNCA.runDFS(ToIDom, 0, DescendBelow, 0);
     LLVM_DEBUG(dbgs() << "\tRunning Semi-NCA\n");
     SNCA.runSemiNCA();
@@ -1096,7 +1097,7 @@ template <typename DomTreeT> struct SemiNCAInfo {
       return false;
     };
 
-    SemiNCAInfo SNCA(BUI);
+    SemiNCAInfo SNCA(DT, BUI);
     unsigned LastDFSNum =
         SNCA.runDFS(ToTN->getBlock(), 0, DescendAndCollect, 0);
 
@@ -1630,7 +1631,7 @@ void ApplyUpdates(DomTreeT &DT,
 
 template <class DomTreeT>
 bool Verify(const DomTreeT &DT, typename DomTreeT::VerificationLevel VL) {
-  SemiNCAInfo<DomTreeT> SNCA(nullptr);
+  SemiNCAInfo<DomTreeT> SNCA(DT, nullptr);
 
   // Simplist check is to compare against a new tree. This will also
   // usefully print the old and new trees, if they are different.
diff --git a/llvm/unittests/Support/GenericDomTreeTest.cpp b/llvm/unittests/Support/GenericDomTreeTest.cpp
index f0f87e3a98908..a8da057ffc1de 100644
--- a/llvm/unittests/Support/GenericDomTreeTest.cpp
+++ b/llvm/unittests/Support/GenericDomTreeTest.cpp
@@ -95,13 +95,8 @@ TEST(GenericDomTree, BlockNumbers) {
   // Adding a new node with a higher number is no problem
   NumberedNode *N3 = G.addNode();
   EXPECT_EQ(DT.getNode(N3), nullptr);
-  // ... even if it exceeds getMaxNumber()
-  NumberedNode *N4 = G.addNode();
-  N4->Number = 1000;
-  EXPECT_EQ(DT.getNode(N4), nullptr);
 
   DT.addNewBlock(N3, N1);
-  DT.addNewBlock(N4, N1);
   for (auto &N : G.Nodes)
     EXPECT_EQ(DT.getNode(N.get())->getBlock(), N.get());
 }