FYI, I just posted an RFC on llvm-dev about moving to the new pass manager by default

This switch would actually happen in Clang, although most of the impact is on LLVM itself (hence the llvm-dev posting). But I wanted to mention it to the Clang developers so folks are aware and encourage anyone with concerns to join that discussion:
http://lists.llvm.org/pipermail/llvm-dev/2017-October/118280.html

(Or to leave comments and/or concerns here, I’m happy to respond in either place…)

Note that this would not remove the flag, so the old infrastructure would remain available and a flag away for some time.

-Chandler