Re: [PATCH 2/3] Introduce <pointer_guard.h>, extracted from <sysdep.h>

[DJ Delorie <dj@redhat.com>]

Florian Weimer via Libc-alpha <libc-alpha@sourceware.org> writes:
> +++ b/sysdeps/generic/pointer_guard.h
> +#ifndef __ASSEMBLER__
> +# define PTR_MANGLE(x) (void) (x)
> +# define PTR_DEMANGLE(x) (void) (x)
> +#endif

These macros will always be called standalone, i.e.

       inline uintptr_t demangle_ptr (uintptr_t x)
 	{
 	  PTR_DEMANGLE (x);
 	  return x;
 	}

If that's the expectation, is there a reason why we need to even
reference X at all?  Would an empty declaration be valid?  This should
only affect volatile variables, but those would be referenced shortly
after anyway.  Atomic boundaries aren't an issue as atomics are not used
in the mangling compuation.

I can see having a non-empty body as a valid way to validate the usage,
but in that case something that actually validates it makes more sense
by only accepting a LHS:

> +# define PTR_MANGLE(x) (void) (&(x))

(the non-generic usages should enforce this through their assignments)

Also, is there any case where code could be corrupted by the lack of
overall parens?  I.e. is this better:

> +# define PTR_MANGLE(x) ((void) (x))