Uh, yes, if you guarantee the semantics only when the code explicitly opts in and not by default then semantics will not subtly change, that is the point of my comment
Uh, yes, if you guarantee the semantics only when the code explicitly opts in and not by default then semantics will not subtly change, that is the point of my comment
It's not a change in semantics of compiled code. It is only a change of whether or not the code will compile.
Guaranteeing an optimization that otherwise only might run is a change in semantics. The attribute doesn't allow (in any sensible language) the code to simply not compile because the optimizer doesn't feel like it today (or you compiled with -O0), it forces the compiler to not allocate a stack frame wherever the code fits the structure that makes that definitely possible and fails to compile wherever it doesn't (even if after other optimization passes it happens to fit a structure that makes it possible).