[GHC] #16402: GHC doesnt' notice that (narrowWordFOO (x .&. FOO)) is just `x`.

6 Mar 2019

      #16402: GHC doesnt' notice that (narrowWordFOO (x .&. FOO)) is just `x`.
-------------------------------------+-------------------------------------
           Reporter:  Fuuzetsu       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# OPTIONS_GHC -ddump-simpl -ddump-to-file -dhex-word-literals -O2 #-}
 {-# LANGUAGE TypeApplications #-}
 module NarrowWord where

 import Data.Word
 import Data.Bits

 smallWord_foo :: Word64 -> Word64
 smallWord_foo x = fromIntegral @Word16 $ fromIntegral (x .&. 0xFFFF)

 smallWord_bar :: Word64 -> Word64
 smallWord_bar x = fromIntegral @Word16 $ fromIntegral x

 test :: Bool
 test =
     let w = 72430412501
     in smallWord_foo w == smallWord_bar w
 }}}

 {{{#!hs
 -- RHS size: {terms: 7, types: 3, coercions: 0, joins: 0/0}
 smallWord_bar :: Word64 -> Word64
 [GblId,
  Arity=1,
  Caf=NoCafRefs,
  Str=m,
  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= \ (x_a1so [Occ=Once!] :: Word64) ->
                  case x_a1so of { GHC.Word.W64# x#_a2Mw [Occ=Once] ->
                  GHC.Word.W64# (GHC.Prim.narrow16Word# x#_a2Mw)
                  }}]
 smallWord_bar
   = \ (x_a1so :: Word64) ->
       case x_a1so of { GHC.Word.W64# x#_a2Mw ->
       GHC.Word.W64# (GHC.Prim.narrow16Word# x#_a2Mw)
       }

 -- RHS size: {terms: 9, types: 3, coercions: 0, joins: 0/0}
 smallWord_foo :: Word64 -> Word64
 [GblId,
  Arity=1,
  Caf=NoCafRefs,
  Str=m,
  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= \ (x_a1hU [Occ=Once!] :: Word64) ->
                  case x_a1hU of { GHC.Word.W64# x#_a30a [Occ=Once] ->
                  GHC.Word.W64#
                    (GHC.Prim.narrow16Word# (GHC.Prim.and# x#_a30a
 0xffff##))
                  }}]
 smallWord_foo
   = \ (x_a1hU :: Word64) ->
       case x_a1hU of { GHC.Word.W64# x#_a30a ->
       GHC.Word.W64#
         (GHC.Prim.narrow16Word# (GHC.Prim.and# x#_a30a 0xffff##))
       }

 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
 test :: Bool
 [GblId,
  Caf=NoCafRefs,
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)
          Tmpl= GHC.Types.True}]
 test = GHC.Types.True
 }}}

 For Word8, Word16, Word32, Word64 and Int8, Int16, Int32, Int64, I would
 expect GHC to never produce the {{{and}}}.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16402
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