[GHC] #14226: Common Block Elimination pass doesn't eliminate common blocks

[GHC]

#14226: Common Block Elimination pass doesn't eliminate common blocks
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.2.1
  (CodeGen)                          |             Keywords:  newcomer,
      Resolution:                    |  CodeGen
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9157, #14754     |  Differential Rev(s):  Phab:D3973,
       Wiki Page:                    |  Phab:D3999
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Comment (by michalt):

 Replying to [comment:29 bgamari]:
 > > 1) Fix the approach using alpha renaming by actually checking liveness
 on entry and exit for each variable. (to only consider variables that are
 not live across blocks) The disadvantage is that this would be quite a bit
 more costly.
 >
 > Right, I briefly considered implementing this. I don't expect it will be
 absurdly expensive: all we need to know is which variables are mentioned
 in only one block. However, I do wonder how much this will actually
 improve code. Presumably there are cases where this will be *too*
 conservative.
 >
 > > 2) Don't improve CBE itself, but run it a second time after the
 sinking pass - this might get rid of some of the cases from this ticket
 (e.g., the one from comment:6) The advantage is that this would also fix
 #12915; the disadvantage: this might not be always as effective as option
 1).
 >
 > Mmm, interesting idea. This would certainly be worth a try.

 I did a small experiment with this:
 https://ghc.haskell.org/trac/ghc/ticket/12915#comment:5
 (I'm wondering if we could improve the CBE to be a bit cheaper, e.g., by
 improving hashing)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14226#comment:30>
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