Someone looked at this, could not reproduce it.
You reported that "clang++ 7.3.0 from the port doesn't" have it.
That makes it sound like 7.0 *did* fix the bug. If it is still
a bug, please provide details. If "the port" has a fix, please
send it back upstream so we don't have to duplicate the work.
Using a builtin directly in user code is asking for trouble.
Details of their calling sequences, and even whether they exist
at all, is not guaranteed from one release to the next. Use
an intrinsic or a math function here instead.