• Ömer Sinan Ağacan's avatar
    Don't update ModDetails with CafInfos when opts are disabled · 5800ebfe
    Ömer Sinan Ağacan authored
    This is consistent with the interface file behavior where we omit
    HsNoCafRefs annotations with -fomit-interface-pragmas (implied by -O0).
    
    ModDetails and ModIface are just different representations of the same
    thing, so they really need to be in sync. This patch does the right
    thing and does not need too much explanation, but here's an example of a
    problem not doing this causes in !2842:
    
        -- MyInteger.hs
        module MyInteger
          ( MyInteger (MyInteger)
          , ToMyInteger (toMyInteger)
          ) where
    
        newtype MyInteger = MyInteger Integer
    
        class ToMyInteger a where
          toMyInteger :: a -> MyInteger
    
        instance ToMyInteger Integer where
          toMyInteger = MyInteger {- . succ -}
    
        -- Main.hs
        module Main
          ( main
          ) where
    
        import MyInteger (MyInteger (MyInteger), toMyInteger)
    
        main :: IO ()
        main = do
          let (MyInteger i) = (id . toMyInteger) (41 :: Integer)
          print i
    
    If I build this with -O0, without this fix, we generate a ModDetails with
    accurate LFInfo for toMyInteger (MyInteger.$fToMyIntegerInteger) which says that
    it's a LFReEntrant with arity 1. This means in the use site (Main) we tag the
    value:
    
        R3 = MyInteger.$fToMyIntegerInteger_closure + 1;
        R2 = GHC.Base.id_closure;
        R1 = GHC.Base.._closure;
        Sp = Sp - 16;
        call stg_ap_ppp_fast(R4, R3, R2, R1) args: 24, res: 0, upd: 24;
    
    Now we change the definition by uncommenting the `succ` part and it becomes a thunk:
    
        MyInteger.$fToMyIntegerInteger [InlPrag=INLINE (sat-args=0)]
          :: MyInteger.ToMyInteger GHC.Integer.Type.Integer
        [GblId[DFunId(nt)]] =
            {} \u [] $ctoMyInteger_rEA;
    
    and its LFInfo is now LFThunk. This change in LFInfo makes a difference in the
    use site: we can no longer tag it.
    
    But becuase the interface fingerprint does not change (because ModIface does not
    change) we don't rebuild Main and tag the thunk.
    
    (1.2% increase in allocations when building T12545 on armv7 because we
    generate more code without CafInfos)
    
    Metric Increase:
        T12545
    5800ebfe
Code owners : Ben Gamari
UpdateCafInfos.hs 5.29 KB