V2 [PATCH] ucontext.h: Add __INDIRECT_RETURN

[Florian Weimer]

* H. J. Lu:

> +/* On x86, swapcontext returns via indirect branch when the shadow stack
> +   is enabled.  Define __INDIRECT_RETURN to indicate whether swapcontext
> +   returns via indirect branch.  */
> +#if defined __CET__ && (__CET__ & 2) != 0
> +/* Note: Functions returned via indirect branch returns once.  Without
> +   indirect_return attribute, use returns_twice attribute.  */

I still don't understand the first sentence of the second comment.