[llvm] [NFC][TableGen] Use C++17 nested namespace definitions (PR #161958)

[Rahul Joshi via llvm-commits]

https://github.com/jurahul created https://github.com/llvm/llvm-project/pull/161958

Change NamespaceEmitter to emit nested namespace using C++17 nested namespace definitions.

>From f60366418f53024b4f94644ec05455187f5c8afa Mon Sep 17 00:00:00 2001
From: Rahul Joshi <rjoshi at nvidia.com>
Date: Sat, 4 Oct 2025 07:30:01 -0700
Subject: [PATCH] [NFC][TableGen] Use C++17 nested namespace definitions

Change NamespaceEmitter to emit nested namespace using C++17
nested namespace definitions.
---
 llvm/include/llvm/TableGen/CodeGenHelpers.h | 23 +++++++++++----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/llvm/include/llvm/TableGen/CodeGenHelpers.h b/llvm/include/llvm/TableGen/CodeGenHelpers.h
index 7dca6a051ba75..f9629236b9dfd 100644
--- a/llvm/include/llvm/TableGen/CodeGenHelpers.h
+++ b/llvm/include/llvm/TableGen/CodeGenHelpers.h
@@ -38,28 +38,29 @@ class IfDefEmitter {
 // namespace (empty for anonymous namespace) or nested namespace.
 class NamespaceEmitter {
 public:
-  NamespaceEmitter(raw_ostream &OS, StringRef Name) : OS(OS) {
-    emitNamespaceStarts(Name);
+  NamespaceEmitter(raw_ostream &OS, StringRef Name)
+      : Name(trim(Name).str()), OS(OS) {
+    OS << "namespace " << this->Name << " {\n";
   }
 
   ~NamespaceEmitter() { close(); }
 
   // Explicit function to close the namespace scopes.
   void close() {
-    for (StringRef NS : llvm::reverse(Namespaces))
-      OS << "} // namespace " << NS << "\n";
-    Namespaces.clear();
+    if (!Closed)
+      OS << "} // namespace " << Name << "\n";
+    Closed = true;
   }
 
 private:
-  void emitNamespaceStarts(StringRef Name) {
-    llvm::SplitString(Name, Namespaces, "::");
-    for (StringRef NS : Namespaces)
-      OS << "namespace " << NS << " {\n";
+  // Trim "::" prefix.
+  static StringRef trim(StringRef Name) {
+    Name.consume_front("::");
+    return Name;
   }
-
-  SmallVector<StringRef, 2> Namespaces;
+  std::string Name;
   raw_ostream &OS;
+  bool Closed = false;
 };
 
 } // end namespace llvm