[GHC] #8832: Constant-folding regression wrt `clearBit (bit 0) 0 `

1 Mar 2014


      #8832: Constant-folding regression wrt `clearBit (bit 0) 0 `
------------------------------+--------------------------------------------
       Reporter:  hvr         |             Owner:
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:  7.8.1
      Component:  Compiler    |           Version:  7.8.1-rc2
       Keywords:              |  Operating System:  Unknown/Multiple
   Architecture:              |   Type of failure:  Runtime performance bug
  Unknown/Multiple            |         Test Case:
     Difficulty:  Unknown     |          Blocking:
     Blocked By:              |
Related Tickets:              |
------------------------------+--------------------------------------------
 While implementing `zeroBits` (see [83bd2f5fc7e/base]) I noticed that
 constant folding of the expression `clearBit (bit 0) 0` regressed (and
 improved at the same time) from GHC 7.6.3 to GHC 7.8.1, specifically, the
 following module

 {{{#!haskell
 {-# LANGUAGE CPP #-}

 module M where

 import Data.Bits
 import Data.Int
 import Data.Word

 #define T(s,T) \
 s :: T ; \
 s = clearBit (bit 0) 0 ; \

 T(i,Int)
 T(i8,Int8)
 T(i16,Int16)
 T(i32,Int32)
 T(i64,Int64)

 T(w,Word)
 T(w8,Word8)
 T(w16,Word16)
 T(w32,Word32)
 T(w64,Word64)

 T(z,Integer)
 }}}

 compiled with GHC 7.8.1RC2 results in the following Core output:

 {{{#!haskell
 -- GHC 7.8.1RC2

 i = I# (andI# 1 (notI# 1))

 i8 = I8# 0
 i16 = I16# 0
 i32 = I32# 0
 i64 = I64# 0

 w = W# (__word 0)
 w8 = W8# (__word 0)

 w16 = W16# (__word 0)
 w32 = W32# (__word 0)
 w64 = W64# (__word 0)

 z2 = $w$cbit 0
 z1 = complementInteger z2
 z = andInteger z2 z1
 }}}

 Thus, `i` and `z` are not properly constant-folded in GHC 7.8.1RC2. With
 GHC 7.6.3, however, `i` and `z` were properly folded to `0`:

 {{{#!haskell
 -- GHC 7.6.3

 i = I# 0

 i8 =
   case $fBitsInt8_$cbit i of _ { I8# x#_aDf ->
   case $fBitsInt8_$cbit i of _ { I8# x#1_aDr ->
   I8#
     (word2Int#
        (and#
           (int2Word# x#_aDf)
           (xor# (int2Word# x#1_aDr) (__word 18446744073709551615))))
   }
   }

 i16,i32,i64 -- equivalent to i8

 w = W# (__word 0)

 w8 =
   case $fBitsWord8_$cbit i of _ { W8# x#_aEV ->
   case $fBitsWord8_$cbit i of _ { W8# x#1_aF5 ->
   W8# (and# x#_aEV (xor# x#1_aF5 (__word 255)))
   }
   }

 w16,w32,w64 -- equivalent to w8

 z = __integer 0
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8832
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