Profiling can affect program behaviour (-fprof-late)

I found it quite surprising that turning on profiling could insert thunks into my program which then affected the behaviour when combined with some unsafe functions.

I am going to note the example here in case anyone else finds this confusing:

[matt@nixos:~/ghc-prof]$ cat Box.hs
{-# LANGUAGE MagicHash #-}
module Main where

import GHC.Exts

data Box = Box Any

asBox :: a -> Box
asBox x = Box (unsafeCoerce# x)
{-# NOINLINE asBox #-}

closureSize :: Box -> Int
closureSize (Box x) = I# (closureSize# x)


main = do
  print (closureSize (asBox ()))

With just -prof the program prints 4 and with -fprof-late it prints 5.

[matt@nixos:~/ghc-prof]$ cat Box.hs
{-# LANGUAGE MagicHash #-}
module Main where

import GHC.Exts

data Box = Box Any

asBox :: a -> Box
asBox x = Box (unsafeCoerce# x)
{-# NOINLINE asBox #-}

closureSize :: Box -> Int
closureSize (Box x) = I# (closureSize# x)


main = do
  print (closureSize (asBox ()))

If you look at the STG

Before:

Main.asBox [InlPrag=NOINLINE] :: forall a. a -> Main.Box
[GblId, Arity=1, Unf=OtherCon []] =
    CCS_DONT_CARE {} \r [x_sNk] Main.Box [x_sNk];

after

Main.asBox [InlPrag=NOINLINE] :: forall a. a -> Main.Box
[GblId, Arity=1, Unf=OtherCon []] =
    CCS_DONT_CARE {} \r [x_sNl]
        tick<asBox>
        let {
          sat_sNo [Occ=Once1] :: GHC.Types.Any
          [LclId] =
              CCCS {x_sNl} \u [] scc<asBox> x_sNl;
        } in  Main.Box [sat_sNo];

I don't know why this thunk is necessary but it violates the expectations of users using Box that the value placed into the Box is exactly the one which they passed into the function.

cc @AndreasK