[GHC] #9661: Branchless ==# is compiled to branchy code

3 Oct 2014

      #9661: Branchless ==# is compiled to branchy code
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  Runtime
     Blocked By:                     |  performance bug
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 This started as a comment on #6135, but Jan Stolarek requested that I file
 a separate report. The branchless tests `<#`, `>#`, `<=#`, and `>=#`, and
 their wordy cousins, seem to work properly, but `==#` and `eqWord#` don't.
 If I write

 {{{#!hs
 foo x = tagToEnum# ((x <# 3#) `orI#` (x ># 100#) `orI#`
                     (x ==# 12#) `orI#` (x ==# 15#))
 }}}

 I get (in 7.8.3 and in 7.9)

 {{{#!hs
 IsDigit.foo :: GHC.Prim.Int# -> GHC.Types.Bool
 [GblId,
  Arity=1,
  Caf=NoCafRefs,
  Str=DmdType ,
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=False)
          Tmpl= \ (x3_a2if [Occ=Once!] :: GHC.Prim.Int#) ->
                  case x3_a2if of wild_Xe {
                    __DEFAULT ->
                      GHC.Prim.tagToEnum#
                        @ GHC.Types.Bool
                        (GHC.Prim.orI# (GHC.Prim.<# wild_Xe 3) (GHC.Prim.>#
 wild_Xe 100));
                    12 -> GHC.Types.True;
                    15 -> GHC.Types.True
                  }}]
 IsDigit.foo =
   \ (x3_a2if :: GHC.Prim.Int#) ->
     case x3_a2if of wild_Xe {
       __DEFAULT ->
         GHC.Prim.tagToEnum#
           @ GHC.Types.Bool
           (GHC.Prim.orI# (GHC.Prim.<# wild_Xe 3) (GHC.Prim.># wild_Xe
 100));
       12 -> GHC.Types.True;
       15 -> GHC.Types.True
 }}}

 and branching assembly to match. Jan Stolarek indicates this is probably a
 problem with constant folding.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9661
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler