Compile GHC with -prof to get a stack trace on panic

[Simon Marlow]

I think this has been mentioned before but it's probably not widely known:
if you compile GHC profiled (that is, enable GhcProfiled=YES in your mk/
build.mk), then every panic comes with a stack trace.  Here's one I just
saw:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.1.20161206 for x86_64-unknown-linux):
Ix{Int}.index: Index (65536) out of range ((0,28))
CallStack (from -prof):
  HscTypes.bin_fixities (compiler/main/HscTypes.hs:1050:51-56)
  HscMain.checkOldIface (compiler/main/HscMain.hs:(586,20)-(587,60))
  HscMain.hscIncrementalFrontend (compiler/main/HscMain.hs:(556,1)-(618,81))
  HscMain.hscIncrementalCompile (compiler/main/HscMain.hs:(644,1)-(699,32))
  GHC.withCleanupSession (compiler/main/GHC.hs:(464,1)-(473,27))
  GHC.runGhc (compiler/main/GHC.hs:(439,1)-(444,26))
  GHC.defaultErrorHandler (compiler/main/GHC.hs:(379,1)-(411,7))

To get more detail in the stack trace you need to add

  GhcStage2HcOpts += -fprof-auto-top

Or -fprof-auto, depending on how much detail you want.

Cheers
Simon
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