[llvm] [DomTree] Reduce number of hash table lookups (NFC) (PR #73097)

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/73097

Inside runSemiNCA(), create a direct mapping from node number of node info, so we can save the node number -> node pointer -> node info lookup in many cases.

To allow this in more cases, change Label to a node number instead of node pointer.

I've limited this to runSemiNCA() for now, because we have the convenient property there that no new node infos will be added, so we don't have to worry about pointer invalidation.

This gives a pretty nice compile-time improvement: http://llvm-compile-time-tracker.com/compare.php?from=a3908d33b17cb9655d336039bf6a9bd798930eb4&to=ba6ac65bc746eecefe105a7bd11ad3dcaba0f246&stat=instructions%3Au

>From 2501553c7d05f9a6eb35de117d4660d557defb56 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Tue, 21 Nov 2023 16:53:58 +0100
Subject: [PATCH] [DomTree] Reduce number of hash table lookups (NFC)

Inside runSemiNCA(), create a direct mapping from node number of node
info, so we can save the node number -> node pointer -> node info
lookup in many cases.

To allow this in more case, change Label to a node number instead
of node pointer.

I've limited this to runSemiNCA() for now, because we have the
convenient property there that no new node infos will be added,
so we don't have to worry about pointer invalidation.
---
 .../llvm/Support/GenericDomTreeConstruction.h | 30 +++++++++----------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/llvm/include/llvm/Support/GenericDomTreeConstruction.h b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
index d1adcad339c81b2..188f1b44903b3e5 100644
--- a/llvm/include/llvm/Support/GenericDomTreeConstruction.h
+++ b/llvm/include/llvm/Support/GenericDomTreeConstruction.h
@@ -65,7 +65,7 @@ struct SemiNCAInfo {
     unsigned DFSNum = 0;
     unsigned Parent = 0;
     unsigned Semi = 0;
-    NodePtr Label = nullptr;
+    unsigned Label = 0;
     NodePtr IDom = nullptr;
     SmallVector<NodePtr, 2> ReverseChildren;
   };
@@ -189,8 +189,7 @@ struct SemiNCAInfo {
 
       // Visited nodes always have positive DFS numbers.
       if (BBInfo.DFSNum != 0) continue;
-      BBInfo.DFSNum = BBInfo.Semi = ++LastNum;
-      BBInfo.Label = BB;
+      BBInfo.DFSNum = BBInfo.Semi = BBInfo.Label = ++LastNum;
       NumToNode.push_back(BB);
 
       constexpr bool Direction = IsReverse != IsPostDom;  // XOR.
@@ -237,8 +236,9 @@ struct SemiNCAInfo {
   //
   // For each vertex V, its Label points to the vertex with the minimal sdom(U)
   // (Semi) in its path from V (included) to NodeToInfo[V].Parent (excluded).
-  NodePtr eval(NodePtr V, unsigned LastLinked,
-               SmallVectorImpl<InfoRec *> &Stack) {
+  unsigned eval(NodePtr V, unsigned LastLinked,
+                SmallVectorImpl<InfoRec *> &Stack,
+                ArrayRef<InfoRec *> NumToInfo) {
     InfoRec *VInfo = &NodeToInfo[V];
     if (VInfo->Parent < LastLinked)
       return VInfo->Label;
@@ -247,17 +247,17 @@ struct SemiNCAInfo {
     assert(Stack.empty());
     do {
       Stack.push_back(VInfo);
-      VInfo = &NodeToInfo[NumToNode[VInfo->Parent]];
+      VInfo = NumToInfo[VInfo->Parent];
     } while (VInfo->Parent >= LastLinked);
 
     // Path compression. Point each vertex's Parent to the root and update its
     // Label if any of its ancestors (PInfo->Label) has a smaller Semi.
     const InfoRec *PInfo = VInfo;
-    const InfoRec *PLabelInfo = &NodeToInfo[PInfo->Label];
+    const InfoRec *PLabelInfo = NumToInfo[PInfo->Label];
     do {
       VInfo = Stack.pop_back_val();
       VInfo->Parent = PInfo->Parent;
-      const InfoRec *VLabelInfo = &NodeToInfo[VInfo->Label];
+      const InfoRec *VLabelInfo = NumToInfo[VInfo->Label];
       if (PLabelInfo->Semi < VLabelInfo->Semi)
         VInfo->Label = PInfo->Label;
       else
@@ -270,18 +270,20 @@ struct SemiNCAInfo {
   // This function requires DFS to be run before calling it.
   void runSemiNCA(DomTreeT &DT, const unsigned MinLevel = 0) {
     const unsigned NextDFSNum(NumToNode.size());
+    SmallVector<InfoRec *, 8> NumToInfo = {nullptr};
+    NumToInfo.reserve(NextDFSNum);
     // Initialize IDoms to spanning tree parents.
     for (unsigned i = 1; i < NextDFSNum; ++i) {
       const NodePtr V = NumToNode[i];
       auto &VInfo = NodeToInfo[V];
       VInfo.IDom = NumToNode[VInfo.Parent];
+      NumToInfo.push_back(&VInfo);
     }
 
     // Step #1: Calculate the semidominators of all vertices.
     SmallVector<InfoRec *, 32> EvalStack;
     for (unsigned i = NextDFSNum - 1; i >= 2; --i) {
-      NodePtr W = NumToNode[i];
-      auto &WInfo = NodeToInfo[W];
+      auto &WInfo = *NumToInfo[i];
 
       // Initialize the semi dominator to point to the parent node.
       WInfo.Semi = WInfo.Parent;
@@ -294,7 +296,7 @@ struct SemiNCAInfo {
         if (TN && TN->getLevel() < MinLevel)
           continue;
 
-        unsigned SemiU = NodeToInfo[eval(N, i + 1, EvalStack)].Semi;
+        unsigned SemiU = NumToInfo[eval(N, i + 1, EvalStack, NumToInfo)]->Semi;
         if (SemiU < WInfo.Semi) WInfo.Semi = SemiU;
       }
     }
@@ -304,8 +306,7 @@ struct SemiNCAInfo {
     // Note that the parents were stored in IDoms and later got invalidated
     // during path compression in Eval.
     for (unsigned i = 2; i < NextDFSNum; ++i) {
-      const NodePtr W = NumToNode[i];
-      auto &WInfo = NodeToInfo[W];
+      auto &WInfo = *NumToInfo[i];
       const unsigned SDomNum = NodeToInfo[NumToNode[WInfo.Semi]].DFSNum;
       NodePtr WIDomCandidate = WInfo.IDom;
       while (NodeToInfo[WIDomCandidate].DFSNum > SDomNum)
@@ -325,8 +326,7 @@ struct SemiNCAInfo {
     assert(NumToNode.size() == 1 && "SNCAInfo must be freshly constructed");
 
     auto &BBInfo = NodeToInfo[nullptr];
-    BBInfo.DFSNum = BBInfo.Semi = 1;
-    BBInfo.Label = nullptr;
+    BBInfo.DFSNum = BBInfo.Semi = BBInfo.Label = 1;
 
     NumToNode.push_back(nullptr);  // NumToNode[1] = nullptr;
   }