[commit: ghc] master: Do a late CSE pass (0e5d2b7)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/0e5d2b7442ff9e55837913a53da451fb97417496/ghc

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

commit 0e5d2b7442ff9e55837913a53da451fb97417496
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Fri Jun 1 12:53:41 2018 +0100

    Do a late CSE pass
    
    When investigating something else I found that a condition
    was being re-evaluated in wheel-seive1.  Why, when CSE should
    find it?  Because the opportunity only showed up after
    LiberateCase
    
    This patch adds a late CSE pass. Rather than give it an extra
    flag I do it when (cse && (spec_constr || liberate_case)), so
    roughly speaking it happense with -O2.
    
    In any case, CSE is very cheap.
    
    Nofib results are minor but in the right direction:
    
            Program           Size    Allocs   Runtime   Elapsed  TotalMem
    --------------------------------------------------------------------------------
               anna          -0.1%     -0.0%     0.163     0.163      0.0%
              eliza          -0.1%     -0.4%     0.001     0.001      0.0%
               fft2          -0.1%      0.0%     0.087     0.087      0.0%
               mate          -0.0%     -1.3%     -0.8%     -0.8%      0.0%
          paraffins          -0.0%     -0.1%     +0.9%     +0.9%      0.0%
                pic          -0.0%     -0.1%     0.009     0.009      0.0%
       wheel-sieve1          -0.2%     -0.0%     -0.1%     -0.1%      0.0%
    --------------------------------------------------------------------------------
                Min          -0.6%     -1.3%     -2.4%     -2.4%      0.0%
                Max          +0.0%     +0.0%     +3.8%     +3.8%    +23.8%
     Geometric Mean          -0.0%     -0.0%     +0.2%     +0.2%     +0.2%


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

0e5d2b7442ff9e55837913a53da451fb97417496
 compiler/simplCore/SimplCore.hs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/compiler/simplCore/SimplCore.hs b/compiler/simplCore/SimplCore.hs
index 8884636..d461b99 100644
--- a/compiler/simplCore/SimplCore.hs
+++ b/compiler/simplCore/SimplCore.hs
@@ -321,6 +321,12 @@ getCoreToDo dflags
           (CoreDoPasses [ CoreDoSpecialising
                         , simpl_phase 0 ["post-late-spec"] max_iter]),
 
+        -- LiberateCase can yield new CSE opportunities because it peels
+        -- off one layer of a recursive function (concretely, I saw this
+        -- in wheel-sieve1), and I'm guessing that SpecConstr can too
+        -- And CSE is a very cheap pass. So it seems worth doing here.
+        runWhen ((liberate_case || spec_constr) && cse) CoreCSE,
+
         -- Final clean-up simplification:
         simpl_phase 0 ["final"] max_iter,