When building all world with -O2 I've stuck on wxhaskell (all versions):

[11 of 16] Compiling Graphics.UI.WX.Controls ( src/Graphics/UI/WX/Controls.hs, dist/build/Graphics/UI/WX/Controls.o )

<no location info>:
    ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-unknown-linux):
        allocateRegsAndSpill: Cannot read from uninitialized register
    %vI_s154O

I've stripped it down to minimal complete example (but it significantly changed error message)

ghc: panic! (the 'impossible' happened)
  (GHC version 7.8.2 for x86_64-unknown-linux):
        RegAlloc.Liveness.computeLivenss
    SCCs aren't in reverse dependent order
    bad blockId c1tU
    [NONREC
        c1uc:
                movq $-8,%rbx
                andq $main:M.AlignLeft{v rb}_closure+1,%rbx
                addq $8,%rbp
                jmp *(%rbx),
     NONREC
...