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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

{-# 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:

-- 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:

-- 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

Trac metadata

Trac field	Value
Version	7.8.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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