Re: POINTER_ALIGNED_P (was: Re: CVS commit: src/sys)

[Roland Illig <roland.illig%gmx.de@localhost>]

On 16.02.2021 23:40, Valery Ushakov wrote:
> On Tue, Feb 16, 2021 at 22:56:19 +0100, Roland Illig wrote:
>
> > On 16.02.2021 19:46, Roy Marples wrote:
> > > I suggest we change POINTER_ALIGNED_P to accept the alignment value we
> > > want rather than the bitwise test we supply, like so:
> > >
> > > #define POINTER_ALIGNED_P(p, a) (((uintptr_t)(p) & ((a) - 1)))
> >
> > To make sure that this macro doesn't get broken again, I have written
> > the attached tests.  I will commit them after making sure I got the
> > changes to distrib/sets/lists/tests/mi correct.  All the rest I already
> > tested successfully.
>
> I don't see any proposal to change the meaning of this macro to
> actually require the alignment even for arches without strict
> alignment.  Does the attached test really passes on e.g. x86 where the
> macro is always true?  E.g. this one:
>
> > +	if (POINTER_ALIGNED_P(p + 1, 2))
> > +		atf_tc_fail("p + 1 must not be aligned to 2");

Yes, it does.  That's what the "#undef __NO_STRICT_ALIGNMENT" in the
test is for.

I intentionally placed it between <sys/types.h> (which defines that
macro on x86 and some other platforms) and <sys/param.h> (which uses the
macro to switch between the boring "everything is correctly aligned" and
the more interesting formula suggested here.