code review 4564053: sync/atomic: fix check64

sync/atomic: fix check64

The LDREXD and STREXD instructions require
aligned addresses, and the ARM stack is not
guaranteed to be aligned during the check.
This may cause other problems later (on the ARM
not all 64-bit pointers may be 64-bit aligned)
but at least the check is correct now.

[rsc, 2011-06-02 16:14:48]

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

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

[bradfitz, 2011-06-02 16:30:37]

LGTM in that it fixes the xoom build at least.  I don't claim to understand
it.

On Thu, Jun 2, 2011 at 9:14 AM, <rsc@golang.org> wrote:

> Reviewers: golang-dev_googlegroups.com,
>
> Message:
> Hello golang-dev (cc: golang-dev@googlegroups.com),
>
> I'd like you to review this change to
> https://go.googlecode.com/hg
>
>
> Description:
> sync/atomic: fix check64
>
> Please review this at http://codereview.appspot.com/4564053/
>
> Affected files:
>  M src/pkg/sync/atomic/asm_arm.s
>
>
> Index: src/pkg/sync/atomic/asm_arm.s
> ===================================================================
> --- a/src/pkg/sync/atomic/asm_arm.s
> +++ b/src/pkg/sync/atomic/asm_arm.s
> @@ -87,11 +87,14 @@
>  // which will make uses of the 64-bit atomic operations loop forever.
>  // If things are working, set okLDREXD to avoid future checks.
>  // https://bugs.launchpad.net/qemu/+bug/670883.
> -TEXT   check64<>(SB),7,$8
> +TEXT   check64<>(SB),7,$16
>        MOVW    $10, R1
> +       // 8-aligned stack address scratch space.
> +       MOVW    $8(SP), R3
> +       AND     $~7, R3
>  loop:
> -       LDREXD  (SP), R2
> -       STREXD  R2, (SP), R0
> +       LDREXD  (R3), R2
> +       STREXD  R2, (R3), R0
>        CMP     $0, R0
>        BEQ     ok
>        SUB     $1, R1
>
>
>

[rsc, 2011-06-02 17:13:37]

I expanded the CL description to explain better.

[rsc, 2011-06-02 17:13:53]

*** Submitted as http://code.google.com/p/go/source/detail?r=efb9e78d32e7 ***

sync/atomic: fix check64

The LDREXD and STREXD instructions require
aligned addresses, and the ARM stack is not
guaranteed to be aligned during the check.
This may cause other problems later (on the ARM
not all 64-bit pointers may be 64-bit aligned)
but at least the check is correct now.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/4564053