[lld-macho] Minor clean up: use .find() to check for key existence rather than [], which would create a new entry.

[int3]

Thanks!

[keith]

Thanks I wasn't aware of this, but it looks like this was the only one doing this.

[oontvoo]

In D118945#3295074, @keith wrote:

Thanks I wasn't aware of this, but it looks like this was the only one doing this.

Yeah I hadn't noticed this until I ran this code with a readlock (not a write lock) and got a crash due to concurrent writes.

lld/MachO/Driver.cpp

static InputFile *addFile(StringRef path, ForceLoad forceLoadArchive,
   switch (magic) {
   case file_magic::archive: {
     // Avoid loading archives twice. If the archives are being force-loaded,
     // loading them twice would create duplicate symbol errors. In the
     // non-force-loading case, this is just a minor performance optimization.
     // We don't take a reference to cachedFile here because the
     // loadArchiveMember() call below may recursively call addFile() and
     // invalidate this reference.
-    if (ArchiveFile *cachedFile = loadedArchives[path])
-      return cachedFile;
+    auto entry = loadedArchives.find(path);
+    if (entry != loadedArchives.end())
+      return entry->second;
 
     std::unique_ptr<object::Archive> archive = CHECK(
         object::Archive::create(mbref), path + ": failed to parse archive");
 
     if (!archive->isEmpty() && !archive->hasSymbolTable())
       error(path + ": archive has no index; run ranlib to add one");
 
     auto *file = make<ArchiveFile>(std::move(archive));