Short circuit is guaranteed by the language standard. On top of that, plan9 comes with its own C compiler so the people who wrote this also wrote the compiler used to compile it.
I see, I just have no idea how complete the Plan 9 tools are. Working with in-house domain specific language interpreters or compilers I've twice seen unary minus support missing so I assumed anything was possible when something's not fully commercialized.
This popped up in some other thread recently. It was (still is, I suppose) a standard ANSI C89 compiler with a couple of small extensions and tweaks. Ken Thompson's short overview is here -
