[Haskell-cafe] Where's the case? or The difference between simpl and prep

[Simon Peyton-Jones]

Check out 
http://hackage.haskell.org/trac/ghc/wiki/Commentary/Compiler/HscMain
and the notes at the top of
	 http://darcs.haskell.org/ghc/compiler/coreSyn/CorePrep.lhs

Beyond that I'm happy to help

Simon

|  -----Original Message-----
|  From: haskell-cafe-bounces at haskell.org [mailto:haskell-cafe-
|  bounces at haskell.org] On Behalf Of Tom Ellis
|  Sent: 14 March 2013 20:05
|  To: Haskell Cafe
|  Subject: [Haskell-cafe] Where's the case? or The difference between simpl and
|  prep
|  
|  The -ddump-simpl output below doesn't contain a case corresponding to the
|  seq in sum', but the -ddump-prep does.  Isn't the output from simpl the
|  input to prep?  If so, where does the case reappear from?  If not, how are
|  simpl and prep related?
|  
|  It seems to have something to do with "Str=DmdType SS" but I don't
|  understand.  This seems to come from the IdInfo on the Id which is the
|  binder "Test.sum'" but [1] says that this information is optional so it
|  seems strange that such crucial information would be encoded there.
|  
|  Thanks,
|  
|  Tom
|  
|  [1] http://www.haskell.org/ghc/docs/7.6.2/html/libraries/ghc-
|  7.6.2/IdInfo.html#t:IdInfo
|  
|  
|  % cat Test.hs
|  module Test where
|  
|  sum' :: [Integer] -> Integer -> Integer
|  sum' [] n = n
|  sum' (x:xs) n = n `seq` sum' xs (n + x)
|  % ghc -fforce-recomp -ddump-simpl -O2 Test.hs
|  [1 of 1] Compiling Test             ( Test.hs, Test.o )
|  
|  ==================== Tidy Core ====================
|  Result size = 14
|  
|  Rec {
|  Test.sum' [Occ=LoopBreaker]
|    :: [GHC.Integer.Type.Integer]
|       -> GHC.Integer.Type.Integer -> GHC.Integer.Type.Integer
|  [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType SS]
|  Test.sum' =
|    \ (ds_daw :: [GHC.Integer.Type.Integer])
|      (n_a9J :: GHC.Integer.Type.Integer) ->
|      case ds_daw of _ {
|        [] -> n_a9J;
|        : x_a9K xs_a9L ->
|          Test.sum' xs_a9L (GHC.Integer.Type.plusInteger n_a9J x_a9K)
|      }
|  end Rec }
|  
|  
|  
|  % ghc -fforce-recomp -ddump-prep -O2 Test.hs
|  [1 of 1] Compiling Test             ( Test.hs, Test.o )
|  
|  ==================== CorePrep ====================
|  Result size = 17
|  
|  Rec {
|  Test.sum' [Occ=LoopBreaker]
|    :: [GHC.Integer.Type.Integer]
|       -> GHC.Integer.Type.Integer -> GHC.Integer.Type.Integer
|  [GblId, Arity=2, Caf=NoCafRefs, Str=DmdType SS, Unf=OtherCon []]
|  Test.sum' =
|    \ (ds_saQ :: [GHC.Integer.Type.Integer])
|      (n_saS :: GHC.Integer.Type.Integer) ->
|      case ds_saQ of _ {
|        [] -> n_saS;
|        : x_saW xs_saV ->
|          case GHC.Integer.Type.plusInteger n_saS x_saW
|          of sat_saZ { __DEFAULT ->
|          Test.sum' xs_saV sat_saZ
|          }
|      }
|  end Rec }
|  
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