[GHC] #10120: Unnecessary code duplication from case analysis

27 Feb 2015

      #10120: Unnecessary code duplication from case analysis
-------------------------------------+-------------------------------------
              Reporter:  bgamari     |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1-rc2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
-------------------------------------+-------------------------------------
 Consider a case analysis like this,

 {{{#!hs
 predicate c =
        c == '-' || c == '_' || c == '.' || c == '~' || c == ':'
     || c == '@' || c == '&' || c == '=' || c == '+' || c == '$' || c ==
 ','

 f x | predicate x = do_something
 f x = do_something_else
 main = f 'a'
 }}}

 GHC in some cases appears to produce Core for this example by constructing
 a case analysis on `x`, replicating `do_something` in every branch,

 {{{#!hs
 $wa_r3UQ
   :: GHC.Prim.Char#
      -> GHC.Prim.State# GHC.Prim.RealWorld
      -> (# GHC.Prim.State# GHC.Prim.RealWorld, () #)
 [GblId, Arity=2, Str=DmdType <S,1*U><L,U>]
 $wa_r3UQ =
   \ (ww_s3TD :: GHC.Prim.Char#)
     (w_s3TA [OS=OneShot] :: GHC.Prim.State# GHC.Prim.RealWorld) ->
     case ww_s3TD of _ [Occ=Dead] {
       __DEFAULT -> (# w_s3TA, GHC.Tuple.() #);
       '$' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl2_r3Uv GHC.Types.True w_s3TA;
       '&' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl4_r3Ux GHC.Types.True w_s3TA;
       '+' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl6_r3Uz GHC.Types.True w_s3TA;
       ',' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl8_r3UB GHC.Types.True w_s3TA;
       '-' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl10_r3UD GHC.Types.True w_s3TA;
       '.' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl12_r3UF GHC.Types.True w_s3TA;
       ':' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl14_r3UH GHC.Types.True w_s3TA;
       '=' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl16_r3UJ GHC.Types.True w_s3TA;
       '@' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl18_r3UL GHC.Types.True w_s3TA;
       '_' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl20_r3UN GHC.Types.True w_s3TA;
       '~' ->
         GHC.IO.Handle.Text.hPutStr2
           GHC.IO.Handle.FD.stdout lvl22_r3UP GHC.Types.True w_s3TA
     }
 }}}

 This seems to be sub-optimal for instruction cache efficiency, the number
 of branches in generated machine code, code size, and understandability of
 the resulting Core. I would have expected something like,

 ```#!hs
 f x =
     let p = pred x
     in case p of
           True  -> do_something
           False -> do_something_else
 ```

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10120>
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