linux-kernel - Re: bit fields && data tearing

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Date:	Fri, 05 Sep 2014 08:31:06 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	David Laight <David.Laight@...LAB.COM>,
	"'paulmck@...ux.vnet.ibm.com'" <paulmck@...ux.vnet.ibm.com>
CC:	Jakub Jelinek <jakub@...hat.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Mikael Pettersson <mikpelinux@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Tony Luck <tony.luck@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	Richard Henderson <rth@...ddle.net>, linux-arm@...r.kernel.org
Subject: Re: bit fields && data tearing

[ +cc linux-arm ]

Hi David,

On 09/05/2014 04:30 AM, David Laight wrote:
> I've seen gcc generate 32bit accesses for 16bit structure members on arm.
> It does this because of the more limited range of the offsets for the 16bit access.
> OTOH I don't know if it ever did this for writes - so it may be moot.

Can you recall the particulars, like what ARM config or what code?

I tried an overly-simple test to see if gcc would bump up to the word load for
the 12-bit offset mode, but it stuck with register offset rather than immediate
offset. [I used the compiler options for allmodconfig and a 4.8 cross-compiler.]

Maybe the test doesn't generate enough register pressure on the compiler?

Regards,
Peter Hurley

#define ARRAY_SIZE(x)  (sizeof(x)/sizeof((x)[0]))

struct x {
	long unused[64];
	short b[12];
	int unused2[10];
	short c;
};

void store_c(struct x *p, short a[]) {
	int i;

	for (i = 0; i < ARRAY_SIZE(p->b); i++)
		p->b[i] = a[i];
	p->c = 2;
}


void store_c(struct x *p, short a[]) {
   0:	e1a0c00d 	mov	ip, sp
   4:	e3a03000 	mov	r3, #0
   8:	e92dd800 	push	{fp, ip, lr, pc}
   c:	e24cb004 	sub	fp, ip, #4
	int i;

	for (i = 0; i < ARRAY_SIZE(p->b); i++)
		p->b[i] = a[i];
  10:	e191c0b3 	ldrh	ip, [r1, r3]
  14:	e0802003 	add	r2, r0, r3
  18:	e2822c01 	add	r2, r2, #256	; 0x100
  1c:	e2833002 	add	r3, r3, #2
  20:	e3530018 	cmp	r3, #24
  24:	e1c2c0b0 	strh	ip, [r2]
  28:	1afffff8 	bne	10 <store_c+0x10>
	p->c = 2;
  2c:	e3a03d05 	mov	r3, #320	; 0x140
  30:	e3a02002 	mov	r2, #2
  34:	e18020b3 	strh	r2, [r0, r3]
  38:	e89da800 	ldm	sp, {fp, sp, pc}

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