
#8638: Optimize by demoting "denormalized" Integers (i.e. J# -> S#) ------------------------------+-------------------------------------------- Reporter: hvr | Owner: hvr Type: feature | Status: new request | Milestone: 7.8.1 Priority: normal | Version: 7.7 Component: libraries | Operating System: Unknown/Multiple (other) | Type of failure: Runtime performance bug Keywords: integer- | Test Case: gmp | Blocking: Architecture: | Unknown/Multiple | Difficulty: Unknown | Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- In the course of a recent [discussion on reddit](http://www.reddit.com/r/haskell/comments/1twtvm/the_problem_with_integer/) it was highlighted, that `integer-gmp` doesn't try to demote `J#` to the more efficient `S#` if even though they would fit. The attached proof-of-concept patch introduces a "smart" `J#` constructor which constructs a `S#` instead if possible: {{{ #!hs -- | Demote 'J#' to 'S#' if possible. See also 'smartJ#'. toSmall :: Integer -> Integer toSmall i@(S# _) = i toSmall (J# 0# _) = S# 0# toSmall (J# 1# mb#) | isTrue# (v ># 0#) = S# v where v = indexIntArray# mb# 0# toSmall (J# -1# mb#) | isTrue# (v <# 0#) = S# v where v = negateInt# (indexIntArray# mb# 0#) toSmall i = i -- | Smart 'J#' constructor which tries to construct 'S#' if possible smartJ# :: Int# -> ByteArray# -> Integer smartJ# s# mb# = toSmall (J# s# mb#) }}} And replaces a couple of `J#`-constructions which are likely to produce a `S#`-fitting `Integer`. A `nofib` comparison for vanilla GHC HEAD vs patched GHC HEAD is attached for further discussion. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8638 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler