An example for the duplication please check the divModInteger function from integer-simple GHC.Integer.Type.

The STG (GHC 8.2.2) generated from divModInteger :: Integer -> Integer -> (# Integer, Integer #) contains duplications in a closure binder list.

Using my custom STG printer it looks like:

module GHC.Integer.Type where

using GHC.Prim
using GHC.Tuple
using GHC.Types

GHC.Integer.Type.divModInteger {-083-} =
  closure (F:) (B:
  n.s84123 {-s84123-}
  d.s84124 {-s84124-}) {
  case GHC.Integer.Type.quotRemInteger {-084-}
         n.s84123 {-s84123-}
         d.s84124 {-s84124-}
  of qr.s84125 {-s84125-} {
    GHC.Prim.(#,#) {-86-} ipv.s84126 {-s84126-} ipv1.s84127 {-s84127-} ->
      let $j.s84128 {-s84128-} =
            closure (F:
            d.s84124 {-s84124-}
            ipv.s84126 {-s84126-}
            ipv1.s84127 {-s84127-}
            ipv.s84126 {-s84126-}
            ipv1.s84127 {-s84127-}) (B:
            wild.s84129 {-s84129-}) {
            let $j1.s84130 {-s84130-} =
                  closure (F:
                  d.s84124 {-s84124-}
                  ipv.s84126 {-s84126-}
                  ipv1.s84127 {-s84127-}
                  ipv.s84126 {-s84126-}
                  ipv1.s84127 {-s84127-}
                  wild.s84129 {-s84129-}) (B:
                  wild1.s84131 {-s84131-}) {
                  case _stg_prim_negateInt#
                         wild.s84129 {-s84129-}
                  of sat.s84132 {-s84132-} {
                    DEFAULT ->
                      case _stg_prim_==#
                             wild1.s84131 {-s84131-}
                             sat.s84132 {-s84132-}
                      of sat.s84133 {-s84133-} {
                        DEFAULT ->
                          case _stg_prim_tagToEnum#
                                 sat.s84133 {-s84133-}
                          of wild2.s84134 {-s84134-} {
                            GHC.Types.False {-612-} ->
                              GHC.Prim.(#,#) {-86-}
                                ipv.s84126 {-s84126-}
                                ipv1.s84127 {-s84127-}
                            GHC.Types.True {-645-} ->
                              case GHC.Integer.Type.plusInteger {-066-}
                                     ipv1.s84127 {-s84127-}
                                     d.s84124 {-s84124-}
                              of r'.s84135 {-s84135-} {
                                DEFAULT ->
                                  case GHC.Integer.Type.plusInteger {-066-}
                                         ipv.s84126 {-s84126-}
                                         GHC.Integer.Type.lvl11 {-r50574-}
                                  of q'.s84136 {-s84136-} {
                                    DEFAULT ->
                                      GHC.Prim.(#,#) {-86-}
                                        q'.s84136 {-s84136-}
                                        r'.s84135 {-s84135-}
                                  }
                              }
                          }
                      }
                  }}
               
            in case ipv1.s84127 {-s84127-}
               of wild1.s84137 {-s84137-} {
                 GHC.Integer.Type.S# {-621-} i#.s84138 {-s84138-} ->
                   case _stg_prim_<#
                          i#.s84138 {-s84138-} 0#
                   of sat.s84140 {-s84140-} {
                     DEFAULT ->
                       case _stg_prim_>#
                              i#.s84138 {-s84138-} 0#
                       of sat.s84139 {-s84139-} {
                         DEFAULT ->
                           case _stg_prim_-#
                                  sat.s84139 {-s84139-}
                                  sat.s84140 {-s84140-}
                           of sat.s84141 {-s84141-} {
                             DEFAULT ->
                               $j1.s84130 {-s84130-}
                                 sat.s84141 {-s84141-}
                           }
                       }
                   }
                 GHC.Integer.Type.Jp# {-r5813-} dt.s84142 {-s84142-} ->
                   $j1.s84130 {-s84130-} 1#
                 GHC.Integer.Type.Jn# {-r5814-} dt.s84143 {-s84143-} ->
                   $j1.s84130 {-s84130-} -1#
               }}
         
      in case d.s84124 {-s84124-}
         of wild.s84144 {-s84144-} {
           GHC.Integer.Type.S# {-621-} i#.s84145 {-s84145-} ->
             case _stg_prim_<#
                    i#.s84145 {-s84145-} 0#
             of sat.s84147 {-s84147-} {
               DEFAULT ->
                 case _stg_prim_>#
                        i#.s84145 {-s84145-} 0#
                 of sat.s84146 {-s84146-} {
                   DEFAULT ->
                     case _stg_prim_-#
                            sat.s84146 {-s84146-}
                            sat.s84147 {-s84147-}
                     of sat.s84148 {-s84148-} {
                       DEFAULT ->
                         $j.s84128 {-s84128-}
                           sat.s84148 {-s84148-}
                     }
                 }
             }
           GHC.Integer.Type.Jp# {-r5813-} dt.s84149 {-s84149-} ->
             $j.s84128 {-s84128-} 1#
           GHC.Integer.Type.Jn# {-r5814-} dt.s84150 {-s84150-} ->
             $j.s84128 {-s84128-} -1#
         }
  }}

On Mon, Nov 5, 2018 at 2:08 PM Simon Peyton Jones <simonpj@microsoft.com> wrote:

I don’t think there should be duplicates in either. Do you have a test case that shows duplicates?

 

Simon

 

From: ghc-devs <ghc-devs-bounces@haskell.org> On Behalf Of Csaba Hruska
Sent: 04 November 2018 11:22
To: ghc-devs@haskell.org
Subject: Re: StgRhsClosure freevar and argument name duplicates

 

Is it possible that GHC generates STG with invalid binding semantics for certain cases that the Cmm codegen fix or ignore?

This could explain my observations.

I've checked the Stg linter source (StgLint.hs ; GHC 8.2.2 and github master) and it does not check StgRhsClosure free var and binder list at all.

And the scope checker function (addInScopeVars) does not check for duplicates.

 

Any thoughts?

 

Cheers,

Csaba

 

On Sat, Nov 3, 2018 at 9:53 AM Csaba Hruska <csaba.hruska@gmail.com> wrote:

Hi,

 

Can StgRhsClosure's freevar list ([occ]) or argument list ([bndr]) contain duplicates?

 

Cheers,

Csaba

 

data GenStgRhs bndr occ
  = StgRhsClosure
        CostCentreStack         -- CCS to be attached (default is CurrentCCS)
        StgBinderInfo           -- Info about how this binder is used (see below)
        [occ]                   -- non-global free vars; a list, rather than
                                -- a set, because order is important
        !UpdateFlag             -- ReEntrant | Updatable | SingleEntry
        [bndr]                  -- arguments; if empty, then not a function;
                                -- as above, order is important.
        (GenStgExpr bndr occ)   -- body