On Tue, Mar 20, 2018 at 4:23 PM, Linus Torvalds
[off-list ref] wrote:

Hmm. So thanks to the diseased mind of Martin Uecker, there's a better
test for "__is_constant()":

  /* Glory to Martin Uecker [off-list ref] */
  #define __is_constant(a) \
        (sizeof(int) == sizeof(*(1 ? ((void*)((a) * 0l)) : (int*)1)))

that is actually *specified* by the C standard to work, and doesn't
even depend on any gcc extensions.

Well, it does depend on 'sizeof(*(void *)X)' being 1 and the compiler
not complaining about it, and that sizeof(int) is not 1.

But since we depend on those things in the kernel anyway, that's fine.

                Linus