Re: [GHC] #12022: unsafeShiftL and unsafeShiftR are not marked as INLINE

19 May 2016

      #12022: unsafeShiftL and unsafeShiftR are not marked as INLINE
-------------------------------------+-------------------------------------
        Reporter:  Rufflewind        |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  7.10.3
      Resolution:                    |             Keywords:  performance,
                                     |  inline, bits
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by thomie):

 * failure:  None/Unknown => Runtime performance bug

Comment:

 I was confused at first. From `Data.Bits` in `libraries/base`, it is clear
 that `unsafeShiftL` and `unsafeShiftR` //are// marked `INLINE`:
 {{{
 class Eq a => Bits a where
     ...
     unsafeShiftL            :: a -> Int -> a
     {-# INLINE unsafeShiftL #-}
     x `unsafeShiftL` i = x `shiftL` i

     unsafeShiftR            :: a -> Int -> a
     {-# INLINE unsafeShiftR #-}
     x `unsafeShiftR` i = x `shiftR` i
     ....
 }}}

 But I guess you're talking about these instances, which indeed aren't
 marked `INLINE`:
 {{{
 instance Bits Int where
     ...
     (I# x#) `unsafeShiftL` (I# i#) = I# (x# `uncheckedIShiftL#` i#)
     (I# x#) `unsafeShiftR` (I# i#) = I# (x# `uncheckedIShiftRA#` i#)
     ...
 }}}

 {{{
 instance Bits Word where
     ...
     (W# x#) `unsafeShiftL` (I# i#) = W# (x# `uncheckedShiftL#` i#)
     (W# x#) `unsafeShiftR` (I# i#) = W# (x# `uncheckedShiftRL#` i#)
     ...
 }}}

 There is a `Note` which suggests explicit `INLINE`s aren't necessary,
 except for `rotate`:
 {{{
 {-      Note [Constant folding for rotate]
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The INLINE on the Int instance of rotate enables it to be constant
 folded.  For example:

 ...

 All other Bits instances seem to inline well enough on their
 own to enable constant folding; for example 'shift':
      sumU . mapU (`shift` 3) . replicateU 10000000 $ (7 :: Int)
  goes to:
      Main.$wfold =
        \ (ww_sOb :: Int#) (ww1_sOf :: Int#) ->
          case ww1_sOf of wild_XM {
            __DEFAULT -> Main.$wfold (+# ww_sOb 56) (+# wild_XM 1);
            10000000 -> ww_sOb
          }
 -}
 }}}

 But that `Note` is older (2008) than the commit that introduced
 `unsafeShiftL/R` (f1c593e01d740fde1202f84aa37ad4cc95ec7272, 2011).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12022#comment:2
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler