code review 143050043: runtime: fix GOARM<7 build

runtime: fix GOARM<7 build

Update issue 8690

If liblink determines that the host doesn't support TLS it replaces the MRC call with a BL runtime.tls_read_fallback. The problem is save_g doesn't expect anyone to make any BL calls and hasn't setup its own link register properly so when runtime.tls_read_fallback returns the LR points to save_g, not save_g's caller so the RET at the end of the function turns into an infinite loop.

This fix is only a proof of concept, I think the real fix should go into liblink as its MRC substitution is not as transparent as expected.

[dave_cheney.net, 2014-09-15 03:01:46]

Hello rsc@golang.org, minux@golang.org (cc:
golang-codereviews@googlegroups.com),

I'd like you to review this change to
https://code.google.com/p/go

[dave_cheney.net, 2014-09-16 15:12:31]

On 2014/09/15 03:01:46, dfc wrote:
> Hello mailto:rsc@golang.org, mailto:minux@golang.org (cc:
> mailto:golang-codereviews@googlegroups.com),
> 
> I'd like you to review this change to
> https://code.google.com/p/go

Polite ping. I'd appreciate some feedback and/or suggestions to improve this CL
so I can get the armv5 builder running again

[minux, 2014-09-16 16:46:33]

could we assume that the kernel's tls helper only clobber r0 and r11?

[dave_cheney.net, 2014-09-17 02:16:56]

From memory the kernel helper only clobbers R0.

Would you prefer me to use R11 ?
On 17 Sep 2014 02:46, "minux" <minux@golang.org> wrote:

> could we assume that the kernel's tls helper only clobber r0 and r11?
>

[rsc, 2014-09-18 19:04:27]

if you are going to clobber R12 here please document that setg<> now does that,
and make sure all the setg<> call sites are okay.

[dave_cheney.net, 2014-09-18 19:06:59]

Would you like me to use R11 instead ?
On 19 Sep 2014 05:04, <rsc@golang.org> wrote:

> if you are going to clobber R12 here please document that setg<> now
> does that, and make sure all the setg<> call sites are okay.
>
>
>
> https://codereview.appspot.com/143050043/
>

[dave_cheney.net, 2014-09-19 01:30:14]

Hello rsc@golang.org, minux@golang.org (cc:
golang-codereviews@googlegroups.com),

Please take another look.

[dave_cheney.net, 2014-09-19 01:30:47]

On 2014/09/18 19:04:27, rsc wrote:
> if you are going to clobber R12 here please document that setg<> now does
that,
> and make sure all the setg<> call sites are okay.

PTAL. I've adjusted the patch to use R11 instead.

[rsc, 2014-09-19 01:50:09]

LGTM

[dave_cheney.net, 2014-09-19 02:02:39]

*** Submitted as https://code.google.com/p/go/source/detail?r=cb017773523e ***

runtime: fix GOARM<7 build

Update issue 8690

If liblink determines that the host doesn't support TLS it replaces the MRC call
with a BL runtime.tls_read_fallback. The problem is save_g doesn't expect anyone
to make any BL calls and hasn't setup its own link register properly so when
runtime.tls_read_fallback returns the LR points to save_g, not save_g's caller
so the RET at the end of the function turns into an infinite loop.

This fix is only a proof of concept, I think the real fix should go into liblink
as its MRC substitution is not as transparent as expected.

LGTM=rsc
R=rsc, minux
CC=golang-codereviews
https://codereview.appspot.com/143050043