Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -2539,6 +2539,8 @@
 
   SmallVector<StoredDiagnostic, 4> Result;
   Result.reserve(Diags.size());
+  const FileEntry *CachedFE = nullptr;
+  FileID CachedFID;
   for (const StandaloneDiagnostic &SD : Diags) {
     // Rebuild the StoredDiagnostic.
     if (SD.Filename.empty())
@@ -2546,7 +2548,13 @@
     const FileEntry *FE = FileMgr.getFile(SD.Filename);
     if (!FE)
       continue;
-    FileID FID = SrcMgr.translateFile(FE);
+    FileID FID;
+    if (FE == CachedFE) {
+      FID = CachedFID;
+    } else {
+      CachedFID = FID = SrcMgr.translateFile(FE);
+      CachedFE = FE;
+    }
     SourceLocation FileLoc = SrcMgr.getLocForStartOfFile(FID);
     if (FileLoc.isInvalid())
       continue;