panic in PPC NCG

I get panics in the NCG PPC register allocator while compiling these files:

  rts_dist_HC rts/dist/build/StgStdThunks.dyn_o
  rts_dist_HC rts/dist/build/StgStdThunks.thr_dyn_o
  rts_dist_HC rts/dist/build/StgStdThunks.l_dyn_o
  rts_dist_HC rts/dist/build/StgStdThunks.thr_l_dyn_o
  HC [stage 1] libraries/ghc-prim/dist-install/build/GHC/Classes.o
  HC [stage 1] libraries/ghc-prim/dist-install/build/GHC/CString.o
  HC [stage 1] libraries/ghc-prim/dist-install/build/GHC/Debug.o

The panic message is like this:

ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 7.7.20130405 for powerpc-montavista-linux):
        allocateRegsAndSpill: Cannot read from uninitialized register
    %vI_nff

This makes the bootstapping of PPC cross compiler, ehm, delicate.

There is a comment in compiler/nativeGen/RegAlloc/Linear/Main.hs:756

                Nothing | reading   ->
                   pprPanic "allocateRegsAndSpill: Cannot read from uninitialized register" (ppr r)
                   -- NOTE: if the input to the NCG contains some
                   -- unreachable blocks with junk code, this panic
                   -- might be triggered.  Make sure you only feed
                   -- sensible code into the NCG.  In CmmPipeline we
                   -- call removeUnreachableBlocks at the end for this
                   -- reason.

So we have a 'junk code' issue here.

Any hints how I can debug this?