But is there any reason the Haskell compiler couldn't decide to memoize a function call itself if it determined that it would speed things up? To me that sound

But is there any reason the Haskell compiler couldn't decide to memoize a function call itself if it determined that it would speed things up? To me that sounds hard for the compiler to do heuristically but a Haskell compiler should still have more scope to do optimizations like that than a C compiler has.