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

[via cfe-commits]

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@@ -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);
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PeterChou1 wrote:

Good point, I hadn't considered that this was my clumsy attempt at trying to remove the redundant work done by the ASTVisitor. Since we are undeniably doing some redundant work. if you take a look at the Shape class from the e2e test you'll see that we visit the declaration 3 times once for parsing the initial file and the twice more for each subclass. Is there any other mechanism that prevents this type of behaviour? This essentially what this patch is trying to accomplish 

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