[clang-tools-extra] [clang-doc] Improve performance by adding a short circuit (PR #96809)

[via cfe-commits]

================
@@ -34,13 +43,26 @@ template <typename T> bool MapASTVisitor::mapDecl(const T *D) {
   // If there is an error generating a USR for the decl, skip this decl.
   if (index::generateUSRForDecl(D, USR))
     return true;
+
+  // Prevent Visiting USR twice
+  {
+    std::lock_guard<llvm::sys::Mutex> Guard(USRVisitedGuard);
+    std::string Visited = USR.str().str();
+    if (USRVisited.count(Visited))
+      return true;
+    // We considered a USR to be visited only when its defined
+    if (IsDefinition)
+      USRVisited.insert(Visited);
----------------
PeterChou1 wrote:

My logic was that definition of the USR is parsed last, so when the ASTVisitor visits the definition it would have already parsed every other USR that points to the same declaration so we can safely short circuit, since every other fragments of USR would've been parsed already



https://github.com/llvm/llvm-project/pull/96809