Recover performance loss after PagedVector introduction (#67972)

Author: ktf

clang/include/clang/Basic/SourceManager.h

@@ -724,7 +724,7 @@ class SourceManager : public RefCountedBase<SourceManager> {
   ///
   /// Negative FileIDs are indexes into this table. To get from ID to an index,
   /// use (-ID - 2).
-  llvm::PagedVector<SrcMgr::SLocEntry> LoadedSLocEntryTable;
+  llvm::PagedVector<SrcMgr::SLocEntry, 32> LoadedSLocEntryTable;
 
   /// For each allocation in LoadedSLocEntryTable, we keep the first FileID.
   /// We assume exactly one allocation per AST file, and use that to determine

llvm/include/llvm/ADT/PagedVector.h

@@ -84,7 +84,7 @@ template <typename T, size_t PageSize = 1024 / sizeof(T)> class PagedVector {
     assert(Index / PageSize < PageToDataPtrs.size());
     T *&PagePtr = PageToDataPtrs[Index / PageSize];
     // If the page was not yet allocated, allocate it.
-    if (!PagePtr) {
+    if (LLVM_UNLIKELY(!PagePtr)) {
       PagePtr = Allocator.getPointer()->template Allocate<T>(PageSize);
       // We need to invoke the default constructor on all the elements of the
       // page.