Re: bit fields && data tearing

Peter Hurley
2014-09-05T00:59:32Z

[ +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