Fix incorrect check for running out of source locations.

[ppluzhnikov]

Summary

When CurrentLoadedOffset is less than TotalSize, current code will
trigger unsigned overflow and will not return an "allocation failed"
indicator.

Google ref: b/248613299

[ppluzhnikov]

Richard, could you take a look?

You've added the check in commit 78d81ecfc38c2

[ppluzhnikov]

Richard, ping?

[dblaikie]

Sounds good (guess this is pretty impractical to have a regression test/would require very large files to test?)

[ppluzhnikov]

In D135192#3866294, @dblaikie wrote:

Sounds good (guess this is pretty impractical to have a regression test/would require very large files to test?)

Yea, overflowing this requires a very convoluted setup; I don't think it's practical to have a unit test for this.

Could you commit this change? (I don't have commit rights in LLVM).

Thanks,

clang/lib/Basic/SourceManager.cpp

const SrcMgr::SLocEntry &SourceManager::loadSLocEntry(unsigned Index,
   return LoadedSLocEntryTable[Index];
 }
 
 std::pair<int, SourceLocation::UIntTy>
 SourceManager::AllocateLoadedSLocEntries(unsigned NumSLocEntries,
                                          SourceLocation::UIntTy TotalSize) {
   assert(ExternalSLocEntries && "Don't have an external sloc source");
   // Make sure we're not about to run out of source locations.
-  if (CurrentLoadedOffset - TotalSize < NextLocalOffset)
+  if (CurrentLoadedOffset < TotalSize ||
+      CurrentLoadedOffset - TotalSize < NextLocalOffset) {
     return std::make_pair(0, 0);
+  }
   LoadedSLocEntryTable.resize(LoadedSLocEntryTable.size() + NumSLocEntries);
   SLocEntryLoaded.resize(LoadedSLocEntryTable.size());
   CurrentLoadedOffset -= TotalSize;
   int ID = LoadedSLocEntryTable.size();
   return std::make_pair(-ID - 1, CurrentLoadedOffset);
 }
 
 /// As part of recovering from missing or changed content, produce a