> Firstly, char types alias everything. (I'm not sure why you wrote that?) I know if you cast to char type, it can, but I'm going from a char type. > Secondly, even if you call memcpy(), a compiler doesn't have to do any > copies if it can prove that the union you're reading into doesn't > escape. If the compiler can optimize away memcpy, why memcpy into a union, why not just to the type in question? Or is memcpy into a union special? In other words, if I write: msg p; memcpy(&p, get_bytes(), sizeof p); // assume the size OK if (p.i) // ... Can the memcpy be optimized away, making it similar to the cast version (but not undefined)? >> Putting alignment/padding concerns aside FYI, I meant assume I'm on x86, or hardware that allows unaligned reads, and my struct doesn't have padding issues. For example, sending a pragma packed struct over a socket. >> Another related, maybe more important, question is if GCC sees a >> reinterpet_cast like this (without a may_alias type), is it free to >> discard code or otherwise drastically change it due to the fact that >> it’s undefined by the standard? > > Yes. It may, and it does. At some point, I understand GCC began to optimize more heavily in strict-aliasing opportunities. For future reference, when GCC makes changes like this, are they always mentioned in the release notes or someplace else? Thx! Jay Haynberg
