Re: [GHC] #11565: Restore code to handle '-fmax-worker-args' flag

31 Aug 2016

      #11565: Restore code to handle '-fmax-worker-args' flag
-------------------------------------+-------------------------------------
        Reporter:  slyfox            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by slyfox):

 Here comes minimal example for one direction: heap to stack.

 The trigger is a function with many USED record fields: show in this case.
 In case of DynFlags it's a full (or large) subset of fields used
 in various GHC subsystems.

 {{{#!hs
 -- A.hs
 module A(D) where

 -- like DynFlgs in GHC
 data D = D { f_00, f_01, f_02, f_03, f_04
            , f_10, f_11, f_12, f_13, f_14
            , f_20, f_21, f_22, f_23, f_24
            , f_30, f_31, f_32, f_33, f_34
            , f_40, f_41, f_42, f_43, f_44
            , f_50, f_51, f_52, f_53, f_54

            , g_00, g_01, g_02, g_03, g_04
            , g_10, g_11, g_12, g_13, g_14
            , g_20, g_21, g_22, g_23, g_24
            , g_30, g_31, g_32, g_33, g_34
            , g_40, g_41, g_42, g_43, g_44
            , g_50, g_51, g_52, g_53, g_54

            , h_00, h_01, h_02, h_03, h_04
            , h_10, h_11, h_12, h_13, h_14
            , h_20, h_21, h_22, h_23, h_24
            , h_30, h_31, h_32, h_33, h_34
            , h_40, h_41, h_42, h_43, h_44
            , h_50, h_51, h_52, h_53, h_54

            , i_00, i_01, i_02, i_03, i_04
            , i_10, i_11, i_12, i_13, i_14
            , i_20, i_21, i_22, i_23, i_24
            , i_30, i_31, i_32, i_33, i_34
            , i_40, i_41, i_42, i_43, i_44
            , i_50, i_51, i_52, i_53, i_54 :: Int

            } deriving Show
 }}}

 {{{#!hs
 -- B.hs
 module B (tiny_foo) where

 import qualified A

 tiny_foo :: A.D -> Bool
 tiny_foo d = null (show d)
 }}}

 Let's look at the size of module B on '''-O0''' and '''-O1''' while A is
 compiled -O2:

 '''-O0''', no unboxing happens.
 {{{#!hs
 $ ghc -c -O2 A.hs && ghc -c -O0 B.hs -ddump-stg -fforce-recomp
 compilation IS NOT required

 ==================== STG syntax: ====================
 $trModule1_r3JD :: GHC.Types.TrName
 [GblId, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
     NO_CCS GHC.Types.TrNameS! ["main"#];

 $trModule2_r3P0 :: GHC.Types.TrName
 [GblId, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
     NO_CCS GHC.Types.TrNameS! ["B"#];

 B.$trModule :: GHC.Types.Module
 [GblId, Caf=NoCafRefs, Str=DmdType, Unf=OtherCon []] =
     NO_CCS GHC.Types.Module! [$trModule1_r3JD $trModule2_r3P0];

 B.tiny_foo :: A.D -> GHC.Types.Bool
 [GblId, Arity=1, Str=DmdType, Unf=OtherCon []] =
     \r srt:SRT:[r30 :-> A.$fShowD,
                 rAj :-> Data.Foldable.$fFoldable[]] [d_s3P4]
         let {
           sat_s3P5 [Occ=Once] :: [GHC.Types.Char]
           [LclId, Str=DmdType] =
               \u srt:SRT:[r30 :-> A.$fShowD] [] GHC.Show.show A.$fShowD
 d_s3P4;
         } in  Data.Foldable.null Data.Foldable.$fFoldable[] sat_s3P5;
 }}}

 '''-O1''', unboxing hapened:
 {{{#!hs
 $ ghc -c -O2 A.hs && ghc -c -O1 B.hs -ddump-stg -fforce-recomp
 compilation IS NOT required

 ==================== STG syntax: ====================
 B.$trModule2 :: GHC.Types.TrName
 [GblId, Caf=NoCafRefs, Str=DmdType m1, Unf=OtherCon []] =
     NO_CCS GHC.Types.TrNameS! ["main"#];

 B.$trModule1 :: GHC.Types.TrName
 [GblId, Caf=NoCafRefs, Str=DmdType m1, Unf=OtherCon []] =
     NO_CCS GHC.Types.TrNameS! ["B"#];

 B.$trModule :: GHC.Types.Module
 [GblId, Caf=NoCafRefs, Str=DmdType m, Unf=OtherCon []] =
     NO_CCS GHC.Types.Module! [B.$trModule2 B.$trModule1];

 B.$wtiny_foo [InlPrag=[0]]
   :: GHC.Types.Int
      -> GHC.Types.Int
      -> GHC.Types.Int
 ...
      -> GHC.Types.Int
      -> GHC.Types.Bool
 [GblId,
  Arity=120,
  Str=DmdType
 <L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(
 U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)><L,1*U(U)>,
  Unf=OtherCon []] =
     \r srt:SRT:[r3a :-> A.$w$cshowsPrec] [ww_s47S
                                           ww1_s47T
                                           ww2_s47U
                                           ww3_s47V
                                           ww4_s47W
                                           ww5_s47X
                                           ww6_s47Y
 ...
                                           ww115_s49J
                                           ww116_s49K
                                           ww117_s49L
                                           ww118_s49M
                                           ww119_s49N]
         case
             A.$w$cshowsPrec
                 0#
                 ww_s47S
                 ww1_s47T
                 ww2_s47U
                 ww3_s47V
                 ww4_s47W
                 ww5_s47X
                 ww6_s47Y
                 ww7_s47Z
                 ww8_s480
                 ww9_s481
                 ww10_s482
                 ww11_s483
                 ww12_s484
                 ww13_s485
 ...
                 ww117_s49L
                 ww118_s49M
                 ww119_s49N
                 GHC.Types.[]
         of
         _ [Occ=Dead]
         { [] -> GHC.Types.True [];
           : _ [Occ=Dead] _ [Occ=Dead] -> GHC.Types.False [];
         };

 B.tiny_foo [InlPrag=INLINE[0]] :: A.D -> GHC.Types.Bool
 [GblId,
  Arity=1,
  Str=DmdType
 <S,1*U(1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U),1*U(U))>,
  Unf=OtherCon []] =
     \r srt:SRT:[r47O :-> B.$wtiny_foo] [w_s49R]
         case w_s49R of _ [Occ=Dead] {
           A.D ww1_s49T [Occ=Once]
               ww2_s49U [Occ=Once]
               ww3_s49V [Occ=Once]
               ww4_s49W [Occ=Once]
               ww5_s49X [Occ=Once]
 ...
               ww118_s4bM [Occ=Once]
               ww119_s4bN [Occ=Once]
               ww120_s4bO [Occ=Once] ->
               B.$wtiny_foo
                   ww1_s49T
                   ww2_s49U
                   ww3_s49V
                   ww4_s49W
                   ww5_s49X
                   ww6_s49Y
                   ww7_s49Z
                   ww8_s4a0
                   ww9_s4a1
                   ww10_s4a2
                   ww11_s4a3
                   ww12_s4a4
                   ww13_s4a5
                   ww14_s4a6
                   ww15_s4a7
                   ww16_s4a8
 }}}

 This causes a lot of 'mov' instructions from heap to stack to be generated
 at each callsite.
 In this case it's 9 pages:
 {{{#!hs
 $ ghc -c -O2 A.hs && ghc -c -O1 B.hs -ddump-asm -fforce-recomp
 ...
         movq %rbx,856(%rbp)
         movq 880(%rbp),%rbx
         movq %rbx,864(%rbp)
         movq 888(%rbp),%rbx
         movq %rbx,872(%rbp)
         movq 896(%rbp),%rbx
         movq %rbx,880(%rbp)
         movq 904(%rbp),%rbx
         movq %rbx,888(%rbp)
         movq %rax,896(%rbp)
         movq $GHC.Types.[]_closure+1,904(%rbp)
         addq $-24,%rbp
         jmp A.$w$cshowsPrec_info
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11565#comment:6>
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