If certain compilers provide functionality like __aligned__ or __packed__ it isn't undefined anymore on these particular compilers. I was specific about this being a GCC/clang thing (GCC to be exact, but somebody else confirmed clang).
__aligned__/__packed__ allow you to control alignment and packing, useful for e.g. avoiding false sharing in multithreaded code, and potentially useful if you're doing some of the type aliasing allowed by the standard - namely, type punning to/from char.
Unless __aligned__ or __packed__ is documented as also relaxing the strict aliasing rules - if such documentation exists, I haven't found it - there's still undefined behavior from the strict aliasing violation by type punning size_t <-> uint32_t.
Bad alignment is not the only possible "bad" optimization compilers can apply that relies on this being undefined behavior. For example, they may mistakenly assume a size_t value in memory can be cached in a register, and not reloaded from memory when it calls code that uses only uint32_t* pointers, because in a standards compliant program these cannot be used to modify size_t values.
