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Date: Tue, 07 Apr 2015 15:06:00 -0000
From: Jonathan Wakely <jwakely@redhat.com>
To: Hans-Peter Nilsson <hp@bitrange.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
Message-ID: <20150407150617.GH9755@redhat.com>
References: <54DD19B7.6060401@redhat.com> <alpine.BSF.2.02.1504022240580.40679@arjuna.pair.com> <alpine.BSF.2.02.1504030518160.69548@arjuna.pair.com> <20150403141333.GY9755@redhat.com> <alpine.BSF.2.02.1504052052110.29977@arjuna.pair.com> <20150407094458.GA9755@redhat.com> <alpine.BSF.2.02.1504070641560.16304@arjuna.pair.com> <20150407131252.GB9755@redhat.com> <alpine.BSF.2.02.1504070922410.45243@arjuna.pair.com>
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On 07/04/15 10:51 -0400, Hans-Peter Nilsson wrote:
>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.)

The docs are clear that alignof(s.x) is not related to its position in
struct SoSo: https://gcc.gnu.org/onlinedocs/gcc/Alignment.html

I'm not going to worry about that behaviour changing.