[GHC] #10397: Compiler performance regression 7.6 -> 7.8 in elimCommonBlocks

[GHC]

#10397: Compiler performance regression 7.6 -> 7.8 in elimCommonBlocks
-------------------------------------+-------------------------------------
        Reporter:  TobyGoodwin       |                   Owner:
            Type:  bug               |                  Status:  patch
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |  performance
 Type of failure:  None/Unknown      |            Architecture:
      Blocked By:                    |  Unknown/Multiple
 Related Tickets:                    |               Test Case:  see ticket
                                     |                Blocking:
                                     |  Differential Revisions:  Phab:D892
-------------------------------------+-------------------------------------

Comment (by nomeata):

 And just to share the joy, here a side-by-side of the profile output,
 before and after:
 {{{
           Sat May 16 18:12 2015 Time and Allocation Profiling Rep|
 Sat May 16 18:06 2015 Time and Allocation Profiling Rep
                                                                  |
              ghc-stage2 +RTS -t -p -RTS -B/home/jojo/build/haskel|
 ghc-stage2 +RTS -t -p -RTS -B/home/jojo/build/haskel
                                                                  |
           total time  =      198.47 secs   (198467 ticks @ 1000 u|
 total time  =       14.22 secs   (14225 ticks @ 1000 us
           total alloc = 97,018,068,880 bytes  (excludes profiling|
 total alloc = 14,686,848,016 bytes  (excludes profiling
                                                                  |
   COST CENTRE      MODULE       %time %alloc                     |  COST
 CENTRE      MODULE          %time %alloc
                                                                  |
   elimCommonBlocks CmmPipeline   80.5   69.1                     |
 elimCommonBlocks CmmPipeline      19.9   21.2
   sequenceBlocks   AsmCodeGen    13.6   19.2                     |
 SimplTopBinds    SimplCore        10.6   10.2
   SimplTopBinds    SimplCore      0.8    1.5                     |
 pprNativeCode    AsmCodeGen        7.6    8.5
   pprNativeCode    AsmCodeGen     0.7    1.3                     |
 regLiveness      AsmCodeGen        7.6    6.8
   regLiveness      AsmCodeGen     0.5    1.0                     |  StgCmm
 HscMain           5.6    5.3
   RegAlloc         AsmCodeGen     0.4    1.1                     |
 RegAlloc         AsmCodeGen        5.5    7.1
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10397#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler