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

[GHC]

#14226: Common Block Elimination pass doesn't eliminate common blocks
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
  (CodeGen)                          |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by bgamari):

 Well, because we have an extremely narrow definition of "common". Namely,
 to be the same two blocks must be syntactically identical, including local
 register names. I don't know how often this happens in practice, but I'd
 imagine not terribly often.

 I think the criterion that you would ideally want is equivalence up to
 alpha renaming of local registers; unfortunately this would likely be
 considerably more costly to check for. This makes me wonder whether we
 shouldn't just try to avoid duplicating the blocks to begin with (e.g.
 addressing #14222 earlier in the compilation pipeline).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14226#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler