What does the `-O99` flag do?

Question

The title is a slight joke.

I just discovered today that ghc accepts any optimization level without complaining. I know that -O2 is supposed to be the maximum optimization level, and that there is a proposed -O3 here, but I was just curious what happens when you mistakenly specify an even higher optimization level. Does ghc default to -O2 or does it not optimize at all?

Answer 1

It looks like it is clamped to be between 0 and 2 in compiler/main/DynFlags.hs:

updOptLevel :: Int -> DynFlags -> DynFlags
-- ^ Sets the 'DynFlags' to be appropriate to the optimisation level
updOptLevel n dfs
  = dfs2{ optLevel = final_n }
  where
   final_n = max 0 (min 2 n)    -- Clamp to 0 <= n <= 2
   ...