GHC doesnt' notice that `narrowWordFOO (x .&. (2^FOO-1))` is just `narrowWordFOO x`.

{-# 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

-- RHS size: {terms: 7, types: 3, coercions: 0, joins: 0/0}
smallWord_bar :: Word64 -> Word64
[GblId,
 Arity=1,
 Caf=NoCafRefs,
 Str=<S(S),1*U(U)>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=<S(S),1*U(U)>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.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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