[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 alexbiehl):

 I think I found a reproducer for this:

 Trac14626_1.hs
 {{{
 module Trac14626_1 where

 data Style = UserStyle Int
            | PprDebug

 data SDC = SDC !Style !Int

 defaultUserStyle :: Bool -> Style
 defaultUserStyle True = UserStyle 123
 defaultUserStyle False = PprDebug
 }}}

 Trac14626_2.hs
 {{{
 module Trac14626_2 where

 import Trac14626_1

 f :: Int -> SDC
 f x = SDC (defaultUserStyle (x > 1)) x
 }}}

 Compiling with `ghc Trac14626_1 Trac14626_2 -ddump-simpl -O` results in a
 similar scenario than the one described by Heisenbug:

 {{{
 -- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
 defaultUserStyle2
 defaultUserStyle2 = I# 123#

 -- RHS size: {terms: 2, types: 0, coercions: 0, joins: 0/0}
 defaultUserStyle1
 defaultUserStyle1 = UserStyle defaultUserStyle2

 -- RHS size: {terms: 7, types: 2, coercions: 0, joins: 0/0}
 defaultUserStyle
 defaultUserStyle
   = \ ds_dZ7 ->
       case ds_dZ7 of {
         False -> PprDebug;
         True -> defaultUserStyle1
       }
 }}}

 Our `UserStyle 123` constant has been lifted to top-level, just like in
 Heisenbugs example.

 Now looking at the Core of `f`

 {{{
 f
 f = \ x_a1dk ->
       case x_a1dk of { I# x1_a2gV ->
       case ># x1_a2gV 1# of {
         __DEFAULT -> SDC PprDebug x1_a2gV;
         1# -> SDC defaultUserStyle1 x1_a2gV
       }
       }
 }}}

 (Note how `f` doesn't scrutinise defaultUserStyle1)

 Looking at the CMM for `f` we can see

 {{{
            ...
            if (%MO_S_Le_W64(_s2hT::I64, 1)) goto c2ip; else goto c2is;
        c2ip:
            I64[Hp - 16] = SDC_con_info;
            P64[Hp - 8] = PprDebug_closure+2;
            I64[Hp] = _s2hT::I64;
            R1 = Hp - 15;
            Sp = Sp + 8;
            call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
        c2is:
            I64[Hp - 16] = SDC_con_info;
            P64[Hp - 8] = defaultUserStyle1_closure; -- defaultUserStyle1
 isn't tagged!
            I64[Hp] = _s2hT::I64;
            R1 = Hp - 15;
            Sp = Sp + 8;
            call (P64[Sp])(R1) args: 8, res: 0, upd: 8;

 }}}

 The strange thing: Putting the definitions into one module Core/Stg look
 the same but the CMM correctly tags the closure.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14626#comment:37>
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