[index] Fix performance regression with indexing macros

When using FileIndexRecord with macros, symbol references can be seen
out of source order, which was causing a regression to insert the
symbols into a vector. Instead, we now lazily sort the vector. The
impact is small on most code, but in very large files with many macro
references (M) near the beginning of the file followed by many decl
references (D) it was O(M*D). A particularly bad protobuf-generated
header was observed with a 100% regression in practice.

rdar://78628133

Author: benlangmuir

clang/lib/Index/FileIndexRecord.cpp

@@ -17,37 +17,32 @@
 using namespace clang;
 using namespace clang::index;
 
-static void addOccurrence(std::vector<DeclOccurrence> &Decls,
-                          DeclOccurrence Info) {
-  auto IsNextOccurence = [&]() -> bool {
-    if (Decls.empty())
-      return true;
-    auto &Last = Decls.back();
-    return Last.Offset < Info.Offset;
-  };
-
-  if (IsNextOccurence()) {
-    Decls.push_back(std::move(Info));
-    return;
+ArrayRef<DeclOccurrence>
+FileIndexRecord::getDeclOccurrencesSortedByOffset() const {
+  if (!IsSorted) {
+    llvm::stable_sort(Decls,
+                      [](const DeclOccurrence &A, const DeclOccurrence &B) {
+                        return A.Offset < B.Offset;
+                      });
+    IsSorted = true;
   }
-
-  // We keep Decls in order as we need to access them in this order in all cases.
-  auto It = llvm::upper_bound(Decls, Info);
-  Decls.insert(It, std::move(Info));
+  return Decls;
 }
 
 void FileIndexRecord::addDeclOccurence(SymbolRoleSet Roles, unsigned Offset,
                                        const Decl *D,
                                        ArrayRef<SymbolRelation> Relations) {
   assert(D->isCanonicalDecl() &&
          "Occurrences should be associated with their canonical decl");
-  addOccurrence(Decls, DeclOccurrence(Roles, Offset, D, Relations));
+  IsSorted = false;
+  Decls.emplace_back(Roles, Offset, D, Relations);
 }
 
 void FileIndexRecord::addMacroOccurence(SymbolRoleSet Roles, unsigned Offset,
                                         const IdentifierInfo *Name,
                                         const MacroInfo *MI) {
-  addOccurrence(Decls, DeclOccurrence(Roles, Offset, Name, MI));
+  IsSorted = false;
+  Decls.emplace_back(Roles, Offset, Name, MI);
 }
 
 void FileIndexRecord::removeHeaderGuardMacros() {

clang/lib/Index/FileIndexRecord.h

@@ -27,14 +27,13 @@ class FileIndexRecord {
 private:
   FileID FID;
   bool IsSystem;
-  std::vector<DeclOccurrence> Decls;
+  mutable bool IsSorted = false;
+  mutable std::vector<DeclOccurrence> Decls;
 
 public:
   FileIndexRecord(FileID FID, bool IsSystem) : FID(FID), IsSystem(IsSystem) {}
 
-  ArrayRef<DeclOccurrence> getDeclOccurrencesSortedByOffset() const {
-    return Decls;
-  }
+  ArrayRef<DeclOccurrence> getDeclOccurrencesSortedByOffset() const;
 
   FileID getFileID() const { return FID; }
   bool isSystem() const { return IsSystem; }