Cache FileID when translating diagnostics in PCH files

[erikjv]

Summary

Modules/preambles/PCH files can contain diagnostics, which, when used,
are added to the current ASTUnit. For that to work, they are translated
to use the current FileManager's FileIDs. When the entry is not the
main file, all local source locations will be checked by a linear
search. Now this is a problem, when there are lots of diagnostics (say,

25000. and lots of local source locations (say, 440000), and end up

taking seconds when using such a preamble.

The fix is to cache the last FileID, because many subsequent diagnostics
refer to the same file. This reduces the time spent in
ASTUnit::TranslateStoredDiagnostics from seconds to a few milliseconds
for files with many slocs/diagnostics.

This fixes PR31353.

[ahatanak]

lib/Frontend/ASTUnit.cpp

2551–2554

Since FID is always equal to CachedFID, I think you can simplify this a bit by removing FID and using just CachedFID.

[bkramer]

lg

lib/Frontend/ASTUnit.cpp

void ASTUnit::TranslateStoredDiagnostics(
                           SmallVectorImpl<StoredDiagnostic> &Out) {
   // Map the standalone diagnostic into the new source manager. We also need to
   // remap all the locations to the new view. This includes the diag location,
   // any associated source ranges, and the source ranges of associated fix-its.
   // FIXME: There should be a cleaner way to do this.
 
   SmallVector<StoredDiagnostic, 4> Result;
   Result.reserve(Diags.size());
+  const FileEntry *PreviousFE = nullptr;
+  FileID FID;
   for (const StandaloneDiagnostic &SD : Diags) {
     // Rebuild the StoredDiagnostic.
     if (SD.Filename.empty())
       continue;
     const FileEntry *FE = FileMgr.getFile(SD.Filename);
     if (!FE)
       continue;
-    FileID FID = SrcMgr.translateFile(FE);
+    if (FE != PreviousFE) {
+      FID = SrcMgr.translateFile(FE);
+      PreviousFE = FE;
+    }
     SourceLocation FileLoc = SrcMgr.getLocForStartOfFile(FID);
     if (FileLoc.isInvalid())
       continue;
     SourceLocation L = FileLoc.getLocWithOffset(SD.LocOffset);
     FullSourceLoc Loc(L, SrcMgr);
 
     SmallVector<CharSourceRange, 4> Ranges;
     Ranges.reserve(SD.Ranges.size());