[commit: ghc] ghc-7.4: MERGE: Be less aggressive about the result discount (46949a1)

[Ian Lynagh]

Repository : ssh://darcs.haskell.org//srv/darcs/ghc

On branch  : ghc-7.4

http://hackage.haskell.org/trac/ghc/changeset/46949a1236657e40a5df79b83b27ceaa08b3eaf5

>---------------------------------------------------------------

commit 46949a1236657e40a5df79b83b27ceaa08b3eaf5
Author: Ian Lynagh <igloo at earth.li>
Date:   Wed May 30 10:43:07 2012 +0100

    MERGE: Be less aggressive about the result discount
    
    Merge of:
        commit 4fa3f16ddb9fa8e5d59bde5354918a39e0430a74
        Author: Simon Peyton Jones <simonpj at microsoft.com>
        Date:   Mon May 28 17:33:42 2012 +0100
    
    Be less aggressive about the result discount
    
    This patch fixes Trac #6099 by reducing the result discount in CoreUnfold.conSize.
    See Note [Constructor size and result discount] in CoreUnfold.
    
    The existing version is definitely too aggressive. Simon M found it an
    "unambiguous win" but it is definitely what led to the bloat. In a function
    with a lot of case branches, all returning a constructor, the discount could
    grow arbitrarily large.
    
    I also had to increase the -funfolding-creation-threshold from 450 to 750,
    otherwise some functions that should inline simply never get an unfolding.
    (The massive result discount was allow the unfolding to appear before.)
    
    The nofib results are these, picking a handful of outliers to show.
    
            Program           Size    Allocs   Runtime   Elapsed  TotalMem
    --------------------------------------------------------------------------------
             fulsom          -0.5%     -1.6%     -2.8%     -2.6%    +31.1%
           maillist          -0.2%     -0.0%      0.09      0.09     -3.7%
             mandel          -0.4%     +6.6%      0.12      0.12     +0.0%
           nucleic2          -0.2%    +18.5%      0.11      0.11     +0.0%
            parstof          -0.4%     +4.0%      0.00      0.00     +0.0%
    --------------------------------------------------------------------------------
                Min          -0.9%     -1.6%    -19.7%    -19.7%     -3.7%
                Max          +0.3%    +18.5%     +2.7%     +2.7%    +31.1%
     Geometric Mean          -0.3%     +0.4%     -3.0%     -3.0%     +0.2%
    
    Turns out that nucleic2 has a function
      Main.$wabsolute_pos =
        \ (ww_s4oj :: Types.Tfo) (ww1_s4oo :: Types.FloatT)
          (ww2_s4op :: Types.FloatT) (ww3_s4oq :: Types.FloatT) ->
          case ww_s4oj
          of _
          { Types.Tfo a_a1sS b_a1sT c_a1sU d_a1sV e_a1sW f_a1sX g_a1sY h_a1sZ i_a1t0 tx_a1t1 ty_a1t2 tz_a1t3 ->
          (# case ww1_s4oo of _ { GHC.Types.F# x_a2sO ->
             case a_a1sS of _ { GHC.Types.F# y_a2sS ->
             case ww2_s4op of _ { GHC.Types.F# x1_X2y9 ->
             case d_a1sV of _ { GHC.Types.F# y1_X2yh ->
             case ww3_s4oq of _ { GHC.Types.F# x2_X2yj ->
             case g_a1sY of _ { GHC.Types.F# y2_X2yr ->
             case tx_a1t1 of _ { GHC.Types.F# y3_X2yn ->
             GHC.Types.F#
               (GHC.Prim.plusFloat#
                  (GHC.Prim.plusFloat#
                     (GHC.Prim.plusFloat#
                        (GHC.Prim.timesFloat# x_a2sO y_a2sS)
                        (GHC.Prim.timesFloat# x1_X2y9 y1_X2yh))
                     (GHC.Prim.timesFloat# x2_X2yj y2_X2yr))
                  y3_X2yn)
             } } }}}}},
    
            <similar>,
            <similar> )
    
    This is pretty big, but inlining it does get rid of that F# allocation.
    But we'll also get rid of it with deep CPR: Trac #2289. For now we just
    accept the change.

 compiler/coreSyn/CoreUnfold.lhs |   57 ++++++++++++++++++++++----------------
 compiler/main/StaticFlags.hs    |    7 ++++-
 2 files changed, 39 insertions(+), 25 deletions(-)


Diff suppressed because of size. To see it, use:

    git show 46949a1236657e40a5df79b83b27ceaa08b3eaf5