From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <iain@sandoe.co.uk>
Received: from smtp002.apm-internet.net (smtp002.apm-internet.net [85.119.248.221])
	by sourceware.org (Postfix) with ESMTPS id 461943858D28
	for <gcc@gcc.gnu.org>; Sun,  4 Sep 2022 13:32:59 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 461943858D28
Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=sandoe.co.uk
Authentication-Results: sourceware.org; spf=none smtp.mailfrom=sandoe.co.uk
Received: (qmail 65763 invoked from network); 4 Sep 2022 13:32:57 -0000
X-APM-Out-ID: 16622983776576
X-APM-Authkey: 257869/1(257869/1) 5
Received: from unknown (HELO ?192.168.1.95?) (81.138.1.83)
  by smtp002.apm-internet.net with SMTP; 4 Sep 2022 13:32:57 -0000
From: Iain Sandoe <iain@sandoe.co.uk>
Content-Type: text/plain;
	charset=utf-8
Content-Transfer-Encoding: quoted-printable
Mime-Version: 1.0 (Mac OS X Mail 13.4 \(3608.120.23.2.7\))
Subject: An odd case with structure field alignment.
Message-Id: <88B0BAB2-A649-406E-B536-EE18431858F1@sandoe.co.uk>
Date: Sun, 4 Sep 2022 14:32:56 +0100
To: GCC Development <gcc@gcc.gnu.org>
X-Mailer: Apple Mail (2.3608.120.23.2.7)
X-Spam-Status: No, score=-8.5 required=5.0 tests=BAYES_00,KAM_COUK,KAM_DMARC_STATUS,KAM_LAZY_DOMAIN_SECURITY,KAM_SHORT,RCVD_IN_DNSWL_LOW,SPF_HELO_NONE,SPF_NONE,TXREP,T_SCC_BODY_TEXT_LINE autolearn=no autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org
List-Id: <gcc.gcc.gnu.org>

Hi,

I am clearly missing something here =E2=80=A6 can someone point out =
where it is?

=
https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Variable-Attributes.html#Variab=
le%20Attributes
in the discussion of applying this to structure fields:

"The aligned attribute can only increase the alignment; but you can =
decrease it by specifying packed as well."

Consider:

struct odd {
  int * __attribute__((aligned(2))) a;
  char c;
};

I would expect, given reading of the information on the aligned =
attribute, that the under-alignment of a would be ignored (since there =
is no packed attribute on either the field or the struct).

However, on x86_64, powerpc64 linux and x86_64, powerpc Darwin, I see =
that the size of the struct is sizeof(pointer) + 2 and the alignment is =
2.

OTOH:

struct OK {
  int  __attribute__((aligned(2))) a;
  char c;
};

behaves as expected (the under-alignment is ignored, silently).

as does this=E2=80=A6

struct maybe {
  int *a  __attribute__((aligned(2)));
  char c;
};

* the type of the pointer does not seem to be relevant (i.e. AFAICT the =
behaviour is the same for char * etc.)

Is there some special rule about pointers that I have not found ?

[it=E2=80=99s making an ABI mismatch with clang, which treats the int * =
as expected from the documentation quoted above]

cheers
Iain