From: James Hogan <james.hogan@imgtec•com>
To: David Laight <David.Laight@ACULAB•COM>
Cc: "devel@driverdev•osuosl.org" <devel@driverdev•osuosl.org>,
"andreas.dilger@intel•com" <andreas.dilger@intel•com>,
Chen Gang <gang.chen.5i5j@gmail•com>,
Greg KH <gregkh@linuxfoundation•org>,
"bergwolf@gmail•com" <bergwolf@gmail•com>,
"linux-kernel@vger•kernel.org" <linux-kernel@vger•kernel.org>,
"linux-metag@vger•kernel.org" <linux-metag@vger•kernel.org>,
"oleg.drokin@intel•com" <oleg.drokin@intel•com>,
"jacques-charles.lafoucriere@cea•fr"
<jacques-charles.lafoucriere@cea•fr>,
Antonio Quartulli <antonio@meshcoding•com>,
netdev <netdev@vger•kernel.org>,
"jinshan.xiong@intel•com" <jinshan.xiong@intel•com>,
David Miller <davem@davemloft•net>,
'Dan Carpenter' <dan.carpenter@oracle•com>
Subject: Re: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union
Date: Mon, 3 Feb 2014 11:02:58 +0000 [thread overview]
Message-ID: <52EF7762.2060809@imgtec.com> (raw)
In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6D0F6B777A@AcuExch.aculab.com>
On 03/02/14 10:35, David Laight wrote:
> From: James Hogan
>> On 03/02/14 10:05, David Laight wrote:
>>> Architectures that define such alignment rules are a right PITA.
>>> You either need to get the size to 2 without using 'packed', or
>>> just not define such structures.
>>> It is worth seeing if adding aligned(2) will change the size - I'm
>>> not sure.
>>
>> __aligned(2) alone doesn't seem to have any effect on sizeof() or
>> __alignof__() unless it is accompanied by __packed. x86_64 is similar in
>> that respect (it just packs sanely in the first place).
>>
>> Combining __packed with __aligned(2) does the trick though (__packed
>> alone sets __aligned(1) which is obviously going to be suboptimal).
>
> Compile some code for a cpu that doesn't support misaligned transfers
> (probably one of sparc, arm, ppc) and see if the compiler generates a
> single 16bit request or two 8 bits ones.
> You don't want the compiler generating multiple byte-sized memory transfers.
Meta is also one of those arches, and according to my quick tests,
__packed alone does correctly make it fall back to byte loads/stores,
but with __packed __aligned(2) it uses 16bit loads/stores. I've also
confirmed that with an ARM toolchain (see below for example).
Cheers
James
input:
#define __aligned(x) __attribute__((aligned(x)))
#define __packed __attribute__((packed))
union a {
short x, y;
} __aligned(2) __packed;
struct b {
short x;
} __aligned(2) __packed;
unsigned int soa = sizeof(union a);
unsigned int aoa = __alignof__(union a);
unsigned int sob = sizeof(struct b);
unsigned int aob = __alignof__(struct b);
void t(struct b *x, union a *y)
{
++x->x;
++y->x;
}
ARM output (-O2):
.cpu arm10tdmi
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 2
.eabi_attribute 30, 2
.eabi_attribute 34, 0
.eabi_attribute 18, 4
.file "alignment4.c"
.text
.align 2
.global t
.type t, %function
t:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
ldrh r3, [r0, #0]
add r3, r3, #1
strh r3, [r0, #0] @ movhi
ldrh r3, [r1, #0]
add r3, r3, #1
strh r3, [r1, #0] @ movhi
bx lr
.size t, .-t
.global aob
.global sob
.global aoa
.global soa
.data
.align 2
.type aob, %object
.size aob, 4
aob:
.word 2
.type sob, %object
.size sob, 4
sob:
.word 2
.type aoa, %object
.size aoa, 4
aoa:
.word 2
.type soa, %object
.size soa, 4
soa:
.word 2
.ident "GCC: (GNU) 4.7.1 20120606 (Red Hat 4.7.1-0.1.20120606)"
.section .note.GNU-stack,"",%progbits
next prev parent reply other threads:[~2014-02-03 11:02 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <52DA4E6A.1000308@gmail.com>
[not found] ` <20140118100547.GS7444@mwanda>
[not found] ` <52DA56C2.5010802@gmail.com>
[not found] ` <20140118142404.GT7444@mwanda>
[not found] ` <52DBA3D4.3090308@gmail.com>
[not found] ` <52DD0EFF.2010305@imgtec.com>
[not found] ` <20140120123045.GV7444@mwanda>
[not found] ` <52DD18A5.1090308@imgtec.com>
[not found] ` <20140120125603.GD4815@mwanda>
[not found] ` <20140120211356.GG4815@mwanda>
[not found] ` <52DE4DA3.7090301@imgtec.com>
2014-01-25 11:55 ` [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union Chen Gang
2014-02-01 13:57 ` Chen Gang
2014-02-03 8:58 ` Dan Carpenter
2014-02-03 10:03 ` Chen Gang
[not found] ` <52EF6965.6040406-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
2014-02-03 11:35 ` Chen Gang
2014-02-03 10:05 ` David Laight
2014-02-03 10:22 ` James Hogan
2014-02-03 10:30 ` Chen Gang
[not found] ` <52EF6DCC.6040807-1AXoQHu6uovQT0dZR+AlfA@public.gmane.org>
2014-02-03 10:35 ` David Laight
2014-02-03 11:02 ` James Hogan [this message]
2014-02-03 11:54 ` David Laight
[not found] ` <063D6719AE5E284EB5DD2968C1650D6D0F6B772B-VkEWCZq2GCInGFn1LkZF6NBPR1lH4CV8@public.gmane.org>
2014-02-03 10:25 ` Chen Gang
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=52EF7762.2060809@imgtec.com \
--to=james.hogan@imgtec$(echo .)com \
--cc=David.Laight@ACULAB$(echo .)COM \
--cc=andreas.dilger@intel$(echo .)com \
--cc=antonio@meshcoding$(echo .)com \
--cc=bergwolf@gmail$(echo .)com \
--cc=dan.carpenter@oracle$(echo .)com \
--cc=davem@davemloft$(echo .)net \
--cc=devel@driverdev$(echo .)osuosl.org \
--cc=gang.chen.5i5j@gmail$(echo .)com \
--cc=gregkh@linuxfoundation$(echo .)org \
--cc=jacques-charles.lafoucriere@cea$(echo .)fr \
--cc=jinshan.xiong@intel$(echo .)com \
--cc=linux-kernel@vger$(echo .)kernel.org \
--cc=linux-metag@vger$(echo .)kernel.org \
--cc=netdev@vger$(echo .)kernel.org \
--cc=oleg.drokin@intel$(echo .)com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox