-mcpu=help doesn't seem to work?

llc -mcpu=help doesn't print any output, just tries to silently read
from standard input. Am I doing something wrong?

Annoyingly enough, you have to give it a bc file as input as well.

Yes, you're right; that's actually just an issue with llc, not with
the underlying library code; programs linked with the library that
provides this help output, correctly produce the output without
requiring an input file. Thanks!

It needs dev/null (or NUL on windows) as the input.

llc -mtriple=avr-atmel-unknown -mcpu=help < dev/null