[GHC] #11784: panic "hscCmmFile: no_mod" with `-v2 and above

[GHC]

#11784: panic "hscCmmFile: no_mod" with `-v2 and above
-------------------------------------+-------------------------------------
           Reporter:  erikd          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Initailly found this on powerpc when debugging #11773, but it also happens
 on x86_64. The command line:

 {{{
 inplace/bin/ghc-stage1  -static  -H64m -O0  -Wall -v2  -Iincludes \
   -Iincludes/dist -Iincludes/dist-derivedconstants/header \
   -Iincludes/dist-ghcconstants/header -Irts -Irts/dist/build \
   -DCOMPILING_RTS -this-unit-id rts -optc-DNOSMP -dcmm-lint -i -irts \
   -irts/dist/build -irts/dist/build/autogen -Irts/dist/build \
   -Irts/dist/build/autogen -O2 -Wnoncanonical-monad-instances -c\
   rts/StgStartup.cmm -o rts/dist/build/StgStartup.o
 }}}

 panics with a message:

 {{{
 ghc-stage1: panic! (the 'impossible' happened)
   (GHC version 8.1.20160402 for powerpc-unknown-linux):
         hscCmmFile: no_mod
 }}}

 The only thing that was added to the command line that is different from
 the normal build command is the `-v2`.

 This is caused by the code (in compiler/main/HscMain.hs):

 {{{
     no_mod = panic "hscCmmFile: no_mod"
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11784>
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