[GHC] #14626: No need to enter a scrutinised value

[GHC]

#14626: No need to enter a scrutinised value
-------------------------------------+-------------------------------------
        Reporter:  heisenbug         |                Owner:  heisenbug
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  performance
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13861            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by heisenbug):

 Replying to [comment:10 simonpj]:
 > The names of the variables should not make any difference!  That's
 bizarre.

 Of course it is not the names, ''per se''! Rather, one of the names
 starting with `ds` is misbehaving in a specific way, that trips over
 `stage2`.

 I built a `quick` compiler with the given revision, that is `stage2`
 compiled with `-O0`, and it behaves well. Testsuite passes without any
 strangeness.

 Then I bisected the sources in `compiler/*` and isolated these two
 (mutually dependent) sources, which when built with `-O1` (and all others
 with `-O0`) break stage 2. Here is what I did (for the record):


 {{{#!sh
 nice make V=1 GhcStage2HcOpts="-O0 -g" -j8
 rm compiler/stage2/build/StgCmmBind.*
 nice make V=1 GhcStage2HcOpts="-O1 -g" -j8
 ./compiler/stage2/build/StgCmmBind.hi
 nice make V=1 GhcStage2HcOpts="-O0 -g" -j8
 }}}

 I'll have a look which differences occur here between `-O0` and `-O1`.
 Both interesting sources are >500 LOC, so it won't be trivial :-)


 >
 > Guessing is usually fruitless; you need data.
 >
 > Did you add that assertionn I suggested?
 >
 > As I say, a stage2 compiler is a huge program.  I urge you to first
 build the libraries and compiler without the change; then switch the
 change on and run the testsuite. Any bugs must be in those little
 programs.  Then nofib.
 >

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14626#comment:11>
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