Re: [libstdc++/65033] Give alignment info to libatomic

[Jonathan Wakely <jwakely at redhat dot com>]

On 12/02/15 13:23 -0800, Richard Henderson wrote:
> When we fixed PR54005, making sure that atomic_is_lock_free returns the same
> value for all objects of a given type, we probably should have changed the
> interface so that we would pass size and alignment rather than size and object
> pointer.
>
> Instead, we decided that passing null for the object pointer would be
> sufficient.  But as this PR shows, we really do need to take alignment into
> account.
>
> The following patch constructs a fake object pointer that is maximally
> misaligned.  This allows the interface to both the builtin and to libatomic to
> remain unchanged.  Which probably makes this back-portable to maintenance
> releases as well.
>
> I believe that for all of our current systems, size_t == uintptr_t, so the
> reinterpret_cast ought not generate warnings.
>
> The test case is problematic, as there's currently no good place to put it.
> The libstdc++ testsuite doesn't have the libatomic library path configured, and
> the libatomic testsuite doesn't have the libstdc++ include paths configured.
> Yet another example where we really need an install tree for testing.  Thoughts?
>
>
> Ok?

OK for trunk.

> r~

> 	* include/bits/atomic_base.h (__atomic_base<T>::is_lock_free): Build
> 	a fake pointer indicating type alignment.
> 	(__atomic_base<T *>::is_lock_free): Likewise.
> 	* include/std/atomic (atomic<T>::is_lock_free): Likewise.