CmmLayoutStack tries to remove a stack check if a function uses no stack space. It knows that Sp < SpLim is always false, but not that Sp >= SpLim is always true. However, the latter can arise when GHC flips the comparison (which it does sometimes).

An example would be the worker function generated for go:

countDown :: Int -> Int
countDown = go 0
    where
        go acc n | n > 0 = go (acc + 1) (n - 1)
                 | otherwise = acc + n