GHC optimisation differences

ghc_api.txt

Output from GhcApi.hs

s1 :: Addr#
[GblId, Caf=NoCafRefs]
s1 = "test"#
s2 :: [Char]
[GblId]
s2 = unpackCString# s1
s3 :: Record
[GblId]
s3 = Record s2
pick :: String
[GblId]
pick = case s3 of { Record ds -> ds }

ghc_exe.txt

Output from ghc -ddump-simpl

-- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
LensOpt.pick1 :: GHC.Prim.Addr#
[GblId,
 Caf=NoCafRefs,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
         WorkFree=True, Expandable=True, Guidance=IF_ARGS [] 20 0}]
LensOpt.pick1 = "test"#

-- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
pick :: String
[GblId,
 Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=True,
         WorkFree=False, Expandable=True, Guidance=IF_ARGS [] 20 0}]
pick = GHC.CString.unpackCString# LensOpt.pick1

GhcApi.hs

-- Program used to compile LensOpt.hs using the GHC API

import GHC
import GhcPlugins
import GHC.Paths

main = runGhc (Just libdir) $ do
    setTargets []
    dflags <- getSessionDynFlags
    setSessionDynFlags dflags{hscTarget = HscNothing}
    res <- compileToCoreSimplified "LensOpt.hs"
    liftIO $ writeFile "lens_api.txt" $ showSDoc dflags $ ppr res

LensOpt.hs

-- Program under test that I would like optimising

{-# OPTIONS_GHC -O2 #-}

module LensOpt(pick) where

import Control.Lens

data Record = Record {_field :: String}

field = lens _field $ \r x -> r{_field=x}

pick = Record "test" ^. field