Subject: Re: [PATCH] drivers: staging: lustre: lustre: include: add "__attribute__((packed))" for the related union

From: James Hogan <[email-masked]>
Date: Mon, 3 Feb 2014 11:02:58 +0000

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