Change in demand analysis results between 7.10.2 and RC1 (not fixed in RC2 and HEAD)

[Ömer Sinan Ağacan]

> Why do you say
>
> |  In our case, we prefer the result in 7.10.2 of course, because that's a
> |  more precise demand and it gives us more opportunities for
> |  optimizations. But I guess this could potentially reveal itself in some
>
> What optimisations do you have in mind?

I just had worker/wrapper in mind. I just realized that <L,A> is actually a
very good demand for W/W, so this new demand is actually better. I was thinking
naively that more strict is better, ignoring the cardinality analysis parts and
redundant argument passing.

2016-02-29 5:10 GMT-05:00 Simon Peyton Jones <simonpj at microsoft.com>:
> See Note [Add demands for strict constructors] in DmdAnal, esp the bit that says
>         If the argument is not used at all in the alternative (i.e. it is
>         Absent), then *don't* add a 'seqDmd'.  If we do, it makes it look used
>         and hence it'll be passed to the worker when it doesn't need to be.
>         Hence the isAbsDmd test in addDataConStrictness.
>
> Why do you say
>
> |  In our case, we prefer the result in 7.10.2 of course, because that's a
> |  more precise demand and it gives us more opportunities for
> |  optimizations. But I guess this could potentially reveal itself in some
>
> What optimisations do you have in mind?
>
> Simon
>
> |  -----Original Message-----
> |  From: Ömer Sinan Ağacan [mailto:omeragacan at gmail.com]
> |  Sent: 27 February 2016 03:13
> |  To: ghc-devs <ghc-devs at haskell.org>; Simon Peyton Jones
> |  <simonpj at microsoft.com>
> |  Cc: Jose Calderon <jmct at jmct.cc>
> |  Subject: Change in demand analysis results between 7.10.2 and RC1 (not
> |  fixed in RC2 and HEAD)
> |
> |  Hi all,
> |
> |  While working on demand analyzer today we realized that there has been
> |  some changes in demand analysis results between GHC 7.10.2 and 8.0-rc2.
> |  Here's a minimal example:
> |
> |      {-# LANGUAGE BangPatterns #-}
> |
> |      module Main where
> |
> |      data Prod a = Prod !a !a
> |
> |      addProd :: Prod Int -> Prod Int -> Prod Int
> |      addProd (Prod i1 i2) (Prod i3 i4) = Prod i1 (i2 + i4)
> |
> |      main = return ()
> |
> |  Compiled with 7.10.2:
> |
> |      addProd :: Prod Int -> Prod Int -> Prod Int
> |      [GblId,
> |       Arity=2,
> |       Caf=NoCafRefs,
> |       Str=DmdType <S(SS),1*U(U,U(U))><S(SS),1*U(1*H,U(U))>m,
> |       ...}}}}]
> |      addProd =
> |        \ (ds_dzH :: Prod Int) (ds1_dzI :: Prod Int) ->
> |          case ds_dzH of _ [Occ=Dead] { Prod i1_an2 i2_an3 ->
> |          case i2_an3 of _ [Occ=Dead] { GHC.Types.I# x_s2B4 ->
> |          case ds1_dzI of _ [Occ=Dead] { Prod i3_an4 i4_an5 ->
> |          case i4_an5 of _ [Occ=Dead] { GHC.Types.I# y_s2B7 ->
> |
> |  https://na01.safelinks.protection.outlook.com/?url=Main.Prod&data=01%7c
> |  01%7csimonpj%40064d.mgd.microsoft.com%7c7b5d6e60d31348506eb108d33f23f6b
> |  f%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=NfCiyeSjPwsWage0KlgkMkQR
> |  jWVexsdlCq0Dla%2f1I10%3d @ Int i1_an2 (GHC.Types.I# (GHC.Prim.+# x_s2B4
> |  y_s2B7))
> |          }
> |          }
> |          }
> |          }
> |
> |  Compiled with 8.0-rc2:
> |
> |      -- RHS size: {terms: 20, types: 17, coercions: 0}
> |      addProd :: Prod Int -> Prod Int -> Prod Int
> |      [GblId,
> |       Arity=2,
> |       Caf=NoCafRefs,
> |       Str=DmdType <S(SS),1*U(U,U(U))><S(LS),1*U(A,U(U))>m,
> |       ...}}}}]
> |      addProd =
> |        \ (ds_dQL :: Prod Int) (ds1_dQM :: Prod Int) ->
> |          case ds_dQL of _ [Occ=Dead] { Prod i1_avS i2_avT ->
> |          case i2_avT of _ [Occ=Dead] { GHC.Types.I# x_s1vO ->
> |          case ds1_dQM of _ [Occ=Dead] { Prod i3_avU i4_avV ->
> |          case i4_avV of _ [Occ=Dead] { GHC.Types.I# y_s1vR ->
> |
> |  https://na01.safelinks.protection.outlook.com/?url=Main.Prod&data=01%7c
> |  01%7csimonpj%40064d.mgd.microsoft.com%7c7b5d6e60d31348506eb108d33f23f6b
> |  f%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=NfCiyeSjPwsWage0KlgkMkQR
> |  jWVexsdlCq0Dla%2f1I10%3d @ Int i1_avS (GHC.Types.I# (GHC.Prim.+# x_s1vO
> |  y_s1vR))
> |          }
> |          }
> |          }
> |          }
> |
> |  To highlight the difference,
> |
> |      GHC 7.10.2:  <S(SS),1*U(U,U(U))><S(SS),1*U(1*H,U(U))>
> |      GHC 8.0-rc2: <S(SS),1*U(U,U(U))><S(LS),1*U(A,U(U))>
> |
> |  (NOTE: Also tried with HEAD and rc1 just now, the results are the same
> |  as rc2)
> |
> |  The demand put on the second argument is more strict in GHC 7.10. Was
> |  that an intentional change? Any ideas on why this might be happening?
> |
> |  In our case, we prefer the result in 7.10.2 of course, because that's a
> |  more precise demand and it gives us more opportunities for
> |  optimizations. But I guess this could potentially reveal itself in some
> |  other situations and make some programs slower? Any ideas?
> |
> |  Thanks..