[GHC] #16351: Extend constant folding operations to bit manipulations

21 Feb 2019

      #16351: Extend constant folding operations to bit manipulations
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           Reporter:  Fuuzetsu       |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           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:
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 In #9136 GHC has greatly improved how it folds expressions for Int/Word
 when the basic arithmetic operations {{{+, -, *}}} are involved.

 I think that somewhat analogously this extend should be extended to binary
 manipulation operations. For example, consider the below snippet:

 {{{#!hs
 {-# OPTIONS_GHC -O2 -ddump-simpl -ddump-to-file -ddump-asm #-}
 module G (bar, baz) where

 import Data.Bits ((.|.))
 import Data.Word (Word16)

 bar :: Word16 -> Int
 bar w = -9223372036854775808 .|. (281474976710656 .|. fromIntegral w)

 baz :: Word16 -> Int
 baz w = -9223372036854775808 + (281474976710656 + fromIntegral w)
 }}}

 Let's peek into the Core:
 {{{#!hs
 bar
   = \ (w_a1hX :: Word16) ->
       case w_a1hX of { GHC.Word.W16# x#_a1Lu ->
       GHC.Types.I#
         (GHC.Prim.orI#
            -9223372036854775808#
            (GHC.Prim.orI# 281474976710656# (GHC.Prim.word2Int# x#_a1Lu)))
       }

 baz
   = \ (w_a1sr :: Word16) ->
       case w_a1sr of { GHC.Word.W16# x#_a1Lu ->
       GHC.Types.I#
         (GHC.Prim.+# -9223090561878065152# (GHC.Prim.word2Int# x#_a1Lu))
       }
 }}}

 Due to #9136, {{{bar}}} gets nicely folded away into a single addition.
 {{{baz}}} however is still in its "naive" form. The above example was
 extracted from an actual program where there may be long such chains of
 {{{orI#}}} and similar operations.

 I think it's worth pointing out that if we peek in the ASM, {{{baz}}}
 seems to get folded away eventually but I don't trust it to always happen
 properly at that level and I don't particularly fancy trawling through the
 ASM output every time.

 https://stackoverflow.com/a/45909278 has some laws we could use. I'm sure
 we could pick in LLVM source &c. too if we want to.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16351
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