------- Comment #23 from rguenth at gcc dot gnu dot org 2006-10-09 08:22 ------- One point to remember is that C does not allow re-using of storage with a different type (which is what PR29272 is about and why that testcase is invalid).
The storage type is either the declared one or the one assigned by virtue of the first assignment (or memcpy). So, int i; float f; memcpy(&f, &i, sizeof(f)); is valid, it doesn't change fs dynamic type but assigns it the bit-pattern of i. What the C++ standard seems to imply is that the storage type of a bunch of memory (or an object with automatic storage) changes on "assignment". So, indeed for C++ re-ordering writes is not allowed, and escaping pointers must be assumed to change dynamic type. So for C++ and 4.2 the best solution looks like to disable type based aliasing completely. For C I'm not sure the behavior the standard mandates is very appealing - at least changing dynamic type via the use of memcpy should be allowed as a GCC extension (like we allow type-punning via the union trick). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29286
