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

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Date:	Thu, 04 Sep 2014 20:59:32 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Laight <David.Laight@...LAB.COM>
CC:	Jakub Jelinek <jakub@...hat.com>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Miroslav Franc <mfranc@...hat.com>,
	Richard Henderson <rth@...ddle.net>,
	linux-alpha@...r.kernel.org
Subject: Re: bit fields && data tearing

[ +cc linux-alpha ]

On 09/04/2014 06:14 PM, H. Peter Anvin wrote:
> On 09/04/2014 02:52 AM, Benjamin Herrenschmidt wrote:
>>
>> Yeah correct, alpha and bytes right ? Is there any other ? That's why I
>> suggested int.
>>
> 
> Even for Alpha it is only the 21064 AFAIK.

For -mcpu=ev5 (21164) and the following test

struct x {
	long a;
	char b;
	char c;
	char d;
	char e;
};

void store_b(struct x *p) {
	p->b = 1;
}

gcc generates:

void store_b(struct x *p) {
   0:	08 00 30 a0 	ldl	t0,8(a0)
   4:	01 f1 3f 44 	andnot	t0,0xff,t0
   8:	01 34 20 44 	or	t0,0x1,t0
   c:	08 00 30 b0 	stl	t0,8(a0)
  10:	01 80 fa 6b 	ret

IOW, rmw on 3 adjacent bytes, which is bad :)
For -mcpu=ev56 (21164A), the generated code is:

void store_b(struct x *p) {
   0:	01 00 3f 20 	lda	t0,1
   4:	08 00 30 38 	stb	t0,8(a0)
   8:	01 80 fa 6b 	ret

which is ok.
I have no idea how prevalent the ev56 is compared to the ev5.
Still we're talking about a chip that came out in 1996.

I still hate split caches though.

Regards,
Peter Hurley

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