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: Tue, 07 Apr 2015 14:51:00 -0000 [thread overview]
Message-ID: <alpine.BSF.2.02.1504070922410.45243@arjuna.pair.com> (raw)
In-Reply-To: <20150407131252.GB9755@redhat.com>
On Tue, 7 Apr 2015, Jonathan Wakely wrote:
> On 07/04/15 06:51 -0400, Hans-Peter Nilsson wrote:
> > On Tue, 7 Apr 2015, Jonathan Wakely wrote:
> > > On 05/04/15 21:07 -0400, Hans-Peter Nilsson wrote:
> > > > 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).
> >
> > I'm more worried about alignof reporting a higher value for a
> > specific object than alignas to be wrong.
>
> That shouldn't be possible because the C++ standard says it's an error
> to use alignas with a less strict alignment than would be used if it
> was omitted, i.e. an error to use alignas with a value less than the
> result alignof would give. However, G++ doesn't reject it (PR65685).
>
> It still won't be possible here, because the alignas value we use is
> not less than alignof(_Tp).
That's not what I meant. My worry is there being a case where
alignof yields a *higher* value than the one that the alignas
specified.
> > Your question quoted just below seems to indicate a similar
> > worry.
>
> I was thinking about cases like this:
>
> struct __attribute__((packed)) Bad {
> char c;
> std::atomic<long long> a;
> };
>
> But G++ ignores the packed attribute here, which is good (Clang
> doesn't seem to ignore it, and mis-aligns the atomic).
I was more thinking of something like:
#include <atomic>
#include <iostream>
using std::cout;
using std::endl;
struct SoSo {
double d;
int x alignas(sizeof(int));
};
SoSo s __attribute__((__aligned__(16)));
int main(void)
{
cout << "alignof(s): " << alignof(s) << endl;
cout << "alignof(s.d): " << alignof(s.d) << endl;
cout << "alignof(s.x): " << alignof(s.x) << endl;
}
in which I fear s.x would get an alignof the same as the s.d or
s, now or after a while, i.e. higher than specified.
(I get for cris-elf at revision 221891:
alignof(s): 16
alignof(s.d): 1
alignof(s.x): 4
which is kind-of-expected except I thought s.d would get the s
alignment so that just leaves it open whether that could
possibly change.)
brgds, H-P
next prev parent reply other threads:[~2015-04-07 14:51 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
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 [this message]
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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