Compiling v8 with recent LLVM fails:

../../src/base/macros.h:290:43: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
  return static_cast<T>(static_cast<T>(0) + x);
                        ~~~~~~~~~~~~~~~~~ ^

this appears to be a warning fairly recently added to LLVM, being treated as an error due to the use of -Werror. See https://reviews.llvm.org/D38186 for the patch which adds it to the LLVM docs, and http://releases.llvm.org/6.0.0/tools/clang/docs/ReleaseNotes.html for the actual llvm 6 release notes which include that text.