code review 5056045: sync/atomic: replace MFENCE with LOCK XADD

sync/atomic: replace MFENCE with LOCK XADD
MFENCE was introduced only on the Pentium4 (SSE2),
while XADD was introduced on the 486.
Fixes issue 2268.

[dvyukov, 2011-09-18 20:22:38]

Hello golang-dev@googlegroups.com (cc: fshahriar@gmail.com,
golang-dev@googlegroups.com),

I'd like you to review this change to
https://go.googlecode.com/hg/

[fhs, 2011-09-18 20:42:23]

I have verified this fixes the issue. Thanks.

fhs

[rsc, 2011-09-19 14:28:33]

LGTM

If this is not more expensive than MFENCE, sure.
Otherwise, I'm not worried about requiring SSE2.
At some point I want to replace the use of x87
with SSE2 anyway.

[dvyukov, 2011-09-19 17:59:51]

They must be roughly the same.
On my Core2 processor MFENCE-based version takes 24.1ns, while XADD - 32.2ns.
However on more recent Xeon E5620 MFENCE - 27.1ns, while XADD - 21.1ns.
Since it is the only thing that prevents Go usage on older machines, I think let
it be XADD for now. Once we have critical mass of things that require SSE2 for
performance, we may fine-tune atomics for latest machines.

[rsc, 2011-09-19 18:02:33]

LGTM

[dvyukov, 2011-09-19 18:04:37]

*** Submitted as http://code.google.com/p/go/source/detail?r=7a7ece494b67 ***

sync/atomic: replace MFENCE with LOCK XADD
MFENCE was introduced only on the Pentium4 (SSE2),
while XADD was introduced on the 486.
Fixes issue 2268.

R=golang-dev, rsc
CC=fshahriar, golang-dev
http://codereview.appspot.com/5056045