http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53278
--- Comment #3 from Ramana Radhakrishnan <ramana at gcc dot gnu.org> 2012-05-08 14:56:18 UTC --- The patch below appears to trigger the issue but there's a fundamental question as to why lower-subreg generates concatns when the documentation suggests that concat and concatn should only be used in declarations but not in the insn stream. http://gcc.gnu.org/ml/gcc-patches/2012-05/msg00423.html