[clang] [clang] SourceManager: Cache offsets for LastFileIDLookup to speed up getFileID (PR #146782)

[Haojian Wu via cfe-commits]

================
@@ -832,13 +835,11 @@ FileID SourceManager::getFileIDLocal(SourceLocation::UIntTy SLocOffset) const {
   unsigned LessIndex = 0;
   // upper bound of the search range.
   unsigned GreaterIndex = LocalSLocEntryTable.size();
-  if (LastFileIDLookup.ID >= 0) {
-    // Use the LastFileIDLookup to prune the search space.
-    if (LocalSLocEntryTable[LastFileIDLookup.ID].getOffset() < SLocOffset)
-      LessIndex = LastFileIDLookup.ID;
-    else
-      GreaterIndex = LastFileIDLookup.ID;
-  }
+  // Use the LastFileIDLookup to prune the search space.
+  if (LastLookupStartOffset < SLocOffset)
+    LessIndex = LastFileIDLookup.ID;
----------------
hokein wrote:

Yes, exactly the old code `LastFileIDLookup.ID >= 0` is always true, it is always executed. 

https://github.com/llvm/llvm-project/pull/146782