[GHC] #14626: No need to enter a scrutinised value

[GHC]

#14626: No need to enter a scrutinised value
-------------------------------------+-------------------------------------
        Reporter:  heisenbug         |                Owner:  heisenbug
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  performance
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13861            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by heisenbug):

 Replying to [comment:33 heisenbug]:
 > It looks ''unary'' to me!

 Interesting, there is a saturated static constructor defined in
 `Outputable`
 {{{
 ==================== Cmm produced by codegen ====================
 2018-01-13 13:02:23.081236 UTC

 [section ""data" . Outputable.defaultUserStyle1_closure" {
      Outputable.defaultUserStyle1_closure:
          const Outputable.PprUser_con_info;
          const Outputable.neverQualify_closure+1;
          const Outputable.AllTheWay_closure+1;
          const Outputable.Uncoloured_closure+1;
          const 3;
  }]


 ==================== Cmm produced by codegen ====================
 2018-01-13 13:02:23.081545 UTC

 [section ""data" . Outputable.defaultUserStyle_closure" {
      Outputable.defaultUserStyle_closure:
          const Outputable.defaultUserStyle_info;
          const 0;
  },
  Outputable.defaultUserStyle_entry() //  [R2]
          { info_tbl: [(chd7,
                        label: Outputable.defaultUserStyle_info
                        rep:HeapRep static { Fun {arity: 1 fun_type:
 ArgSpec 5} })]
            stack_info: arg_space: 8 updfr_space: Just 8
          }
      {offset ...
 }}}

 ''Side question: which optimisation creates this guy?''

 All uses of the `Outputable.defaultUserStyle1_closure` in `Outputable`
 ''are'' tagged with `1`, e.g.:
 {{{
            R1 = Outputable.defaultUserStyle1_closure+1;
 }}}

 But this is not the case in the module `AsmCodeGen`:
 {{{
            P64[Hp - 16] = Outputable.defaultUserStyle1_closure;
 }}}

 This explains why there is no tag sometimes, but the tag being present
 most of the time.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14626#comment:34>
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