[clang] Reduce memory usage in AST parent map generation by lazily checking if nodes have been seen (PR #129934)

[via cfe-commits]

https://github.com/higher-performance updated https://github.com/llvm/llvm-project/pull/129934

>From 6966e56ddfd9a8c8e6a96b2ec6d977a0b7372ad6 Mon Sep 17 00:00:00 2001
From: higher-performance <higher.performance.github at gmail.com>
Date: Wed, 5 Mar 2025 15:49:06 -0500
Subject: [PATCH] Reduce memory usage in AST parent map generation by only
 keeping pointers in the cache

---
 clang/lib/AST/ParentMapContext.cpp | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/clang/lib/AST/ParentMapContext.cpp b/clang/lib/AST/ParentMapContext.cpp
index e9387ec79c373..0f19cc20aaa28 100644
--- a/clang/lib/AST/ParentMapContext.cpp
+++ b/clang/lib/AST/ParentMapContext.cpp
@@ -12,10 +12,11 @@
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/ParentMapContext.h"
-#include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/AST/TemplateBase.h"
+#include "llvm/ADT/SmallPtrSet.h"
 
 using namespace clang;
 
@@ -69,17 +70,22 @@ class ParentMapContext::ParentMap {
       for (; N > 0; --N)
         push_back(Value);
     }
-    bool contains(const DynTypedNode &Value) {
-      return Seen.contains(Value);
+    bool contains(const DynTypedNode &Value) const {
+      const void *Identity = Value.getMemoizationData();
+      assert(Identity);
+      return Dedup.contains(Identity);
     }
     void push_back(const DynTypedNode &Value) {
-      if (!Value.getMemoizationData() || Seen.insert(Value).second)
+      const void *Identity = Value.getMemoizationData();
+      if (!Identity || Dedup.insert(Identity).second) {
         Items.push_back(Value);
+      }
     }
     llvm::ArrayRef<DynTypedNode> view() const { return Items; }
+
   private:
-    llvm::SmallVector<DynTypedNode, 2> Items;
-    llvm::SmallDenseSet<DynTypedNode, 2> Seen;
+    std::vector<DynTypedNode> Items;
+    llvm::SmallPtrSet<const void *, 2> Dedup;
   };
 
   /// Maps from a node to its parents. This is used for nodes that have