[PATCH] D79834: Speed up preamble building by replacing the slow translateFile call by a new, faster isMainFile check

[Jan Korous via Phabricator via cfe-commits]

jkorous added a comment.

IIUC the issue is that `SourceManager::translateFile()` basically consists of two blocks of code:

  // First, check the main file ID, since it is common to look for a
  // location in the main file.
  if (MainFileID.isValid()) {
    bool Invalid = false;
    const SLocEntry &MainSLoc = getSLocEntry(MainFileID, &Invalid);
    if (Invalid)
      return FileID();
  
    if (MainSLoc.isFile()) {
      const ContentCache *MainContentCache =
          MainSLoc.getFile().getContentCache();
      if (MainContentCache && MainContentCache->OrigEntry == SourceFile)
        return MainFileID;
    }
  }

and

    // The location we're looking for isn't in the main file; look
    // through all of the local source locations.
  ...

The comments suggest that the first block is a heuristic related to our case and the second block I would assume being the expensive part. `SourceManager::getFileEntryRefForID` implementation seems similar to the first block.

It makes sense to me to avoid the expensive search. I'm just wondering - how much speedup do we get with caching the value?


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  https://reviews.llvm.org/D79834/new/

https://reviews.llvm.org/D79834