From: Hans-Peter Nilsson <hp@bitrange.com>
To: Jonathan Wakely <jwakely@redhat.com>
Cc: Richard Henderson <rth@redhat.com>,
libstdc++@gcc.gnu.org, gcc-patches@gcc.gnu.org
Subject: Re: [libstdc++/65033] Give alignment info to libatomic
Date: Mon, 06 Apr 2015 01:07:00 -0000 [thread overview]
Message-ID: <alpine.BSF.2.02.1504052052110.29977@arjuna.pair.com> (raw)
In-Reply-To: <20150403141333.GY9755@redhat.com>
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?
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?
> The attached patch against trunk should have the same result with much
> less effort.
>
> It doesn't include the changes to the reinterpret_cast<void *>
> expressions to produce a minimally aligned pointer, but I think this
> is progress, thanks :-)
Progress is good. :)
brgds, H-P
next prev parent reply other threads:[~2015-04-06 1:07 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-02-12 21:23 Richard Henderson
2015-02-18 12:15 ` Jonathan Wakely
2015-03-25 16:22 ` Jonathan Wakely
2015-03-25 18:36 ` Richard Henderson
2015-03-25 18:49 ` Jonathan Wakely
2015-03-25 19:04 ` Richard Henderson
2015-03-26 13:21 ` Jonathan Wakely
2015-03-31 13:41 ` Jonathan Wakely
2015-03-31 14:54 ` Richard Henderson
2015-03-31 15:03 ` Jonathan Wakely
2015-03-31 15:13 ` Richard Henderson
2015-03-31 15:41 ` Jonathan Wakely
2015-04-06 22:59 ` Hans-Peter Nilsson
2015-04-13 4:45 ` patch fix issue 1 with "[libstdc++/65033] Give alignment info to libatomic" Hans-Peter Nilsson
2015-04-13 11:59 ` Jonathan Wakely
2015-04-13 5:59 ` Issue 2 " Hans-Peter Nilsson
2015-04-13 17:53 ` Joseph Myers
2015-03-25 18:39 ` [libstdc++/65033] Give alignment info to libatomic Richard Henderson
2015-04-03 3:04 ` Hans-Peter Nilsson
2015-03-26 11:54 ` Jonathan Wakely
2015-04-03 3:57 ` Hans-Peter Nilsson
2015-04-03 9:25 ` Hans-Peter Nilsson
2015-04-03 14:13 ` Jonathan Wakely
2015-04-03 19:13 ` Richard Henderson
2015-04-07 13:14 ` Jonathan Wakely
2015-04-09 11:17 ` Jonathan Wakely
2015-04-06 1:07 ` Hans-Peter Nilsson [this message]
2015-04-07 9:45 ` Jonathan Wakely
2015-04-07 10:52 ` Hans-Peter Nilsson
2015-04-07 13:12 ` Jonathan Wakely
2015-04-07 14:51 ` Hans-Peter Nilsson
2015-04-07 15:06 ` Jonathan Wakely
2015-04-08 3:58 ` Hans-Peter Nilsson
2015-04-08 9:35 ` Jonathan Wakely
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