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... % 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 }
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@haskell.org [mailto:haskell-cafe- | bounces@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 } | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe
On Thu, Mar 14, 2013 at 10:43:14PM +0000, Simon Peyton-Jones wrote:
| -----Original Message----- | From: Tom Ellis | 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.
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
Thanks Simon. That's lovely code and easy to read. I see that the decision between let and case is made based on the idDemandInfo which specifies whether a value is wanted strictly or not. I presume that the explicit case is removed in the Simpl phase because it is easier for optimisations to work on the Demand value than explicit case statements. Tom
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Simon Peyton-Jones -
Tom Ellis