http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50729
--- Comment #8 from Jorn Wolfgang Rennecke <amylaar at gcc dot gnu.org> 2011-11-19 00:03:56 UTC --- More specifically, the problem appears when innerop has a different signedness than middleop, and the value range of middleop is such that the topmost bit of a double_int is used. E.g. the full value range of a 64 bit long long with 32 bit HOST_WIDE_INT, or a 128 bit integer type with a 64 bit HOST_WIDE_INT. OTOH both the old and the new test will reject some simplifications that would be OK when considering a narrow finaltype, and/or same-size signed/unsigned conversion. The basic problem is that when a signedness change happens, we are no longer necessarily considering a single range, but possibly two.
