This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [libstdc++/65033] Give alignment info to libatomic
- From: Jonathan Wakely <jwakely at redhat dot com>
- To: Hans-Peter Nilsson <hp at bitrange dot com>
- Cc: Richard Henderson <rth at redhat dot com>, libstdc++ at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
- Date: Tue, 7 Apr 2015 10:44:58 +0100
- Subject: Re: [libstdc++/65033] Give alignment info to libatomic
- Authentication-results: sourceware.org; auth=none
- References: <54DD19B7 dot 6060401 at redhat dot com> <alpine dot BSF dot 2 dot 02 dot 1504022240580 dot 40679 at arjuna dot pair dot com> <alpine dot BSF dot 2 dot 02 dot 1504030518160 dot 69548 at arjuna dot pair dot com> <20150403141333 dot GY9755 at redhat dot com> <alpine dot BSF dot 2 dot 02 dot 1504052052110 dot 29977 at arjuna dot pair dot com>
On 05/04/15 21:07 -0400, Hans-Peter Nilsson wrote:
On Fri, 3 Apr 2015, Jonathan Wakely wrote:
On 03/04/15 05:24 -0400, Hans-Peter Nilsson wrote:
> On Thu, 2 Apr 2015, Hans-Peter Nilsson wrote:
> > Why then use __alignof(_M_i) (the object-alignment)
> > instead of _S_alignment (the deduced alas insufficiently
> > increased type-alignment)?
Isn't the object aligned to _S_alignment?
We did specify that with the alignas. Is the alignof always
exactly the same as an alignas, if one is specified? (And will
that not change in a future amendment, standard and/or
implementation?) Either way, is there a test-case to guard all
this?
The language guarantees that's what alignas() does, if the argument is
a valid alignment (which it must be if we derive it from some other
type's alignment).
Those questions wouldn't even be asked if we used _S_alignment
for the fake-pointer too, just as a matter of defensive
programming.
Instead of changing every case in the condition to include sizeof why
not just do it afterwards using sizeof(_Tp), in the _S_alignment
calculation?
Doh.
We know sizeof(_Tp) == sizeof(corresponding integer type) because
that's the whole point of the conditionals! See attachment.
> @@ -216,7 +216,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> is_lock_free() const noexcept
> {
> // Produce a fake, minimally aligned pointer.
> - void *__a = reinterpret_cast<void *>(-__alignof(_M_i));
> + void *__a = reinterpret_cast<void *>(-_S_alignment);
> return __atomic_is_lock_free(sizeof(_M_i), __a);
> }
If _M_i is aligned to _S_alignment then what difference does the
change above make?
It doesn't matter if the value is per-object if we've forced all such
objects to have the same alignment, does it?
Or is it different if a std::atomic<T> is included in some other
struct and the user forces a different alignment on it? I don't think
we really need to support that, users shouldn't be doing that.
Why do we even need to ask those questions, when the patch takes
care of the per-type business without doubt?
Well if we know the object is guaranteed to be correctly aligned we
might not even need a fake, minimally aligned pointer. We could go
back to passing &_M_i or just a null pointer to __atomic_is_lock_free.
The whole point of alignas() is to fix the alignment to a known value.