Fix restore_rt() getter's inline asm not to clobber the x86-64 red zone

Fix restore_rt() getter's inline asm not to clobber the x86-64 red zone

Using "call" in x86-64 inline asm without adjusting the stack pointer
for the 128 byte red zone isn't valid, because it would clobber the
red zone.

This use of "call" comes from the x86-32 version, but it's not
necessary on x86-64 where we can use %rip-relative addressing.

Also relax output constraint from using %rax ('=a') to using any
register ('=r'); the stricter constraint wasn't necessary before.

BUG=none
TEST=none

Committed: https://code.google.com/p/linux-syscall-support/source/detail?r=23

[Mark Seaborn, 2013-08-22 23:39:25]

I haven't run into any problems with this in practice, but I noticed it was a
problem when investigating how inline asm interacts with the red zone.  e.g. See
http://gcc.gnu.org/ml/gcc-help/2011-11/msg00154.html

[jln (very slow on Chromium), 2013-08-29 20:22:03]

lgtm

https://codereview.chromium.org/23072035/diff/1/lss/linux_syscall_support.h
File lss/linux_syscall_support.h (right):

https://codereview.chromium.org/23072035/diff/1/lss/linux_syscall_support.h#n...
lss/linux_syscall_support.h:2036: : "=r" (res)
This should always  have been =r, right?

[Mark Seaborn, 2013-08-31 00:01:58]

https://codereview.chromium.org/23072035/diff/1/lss/linux_syscall_support.h
File lss/linux_syscall_support.h (right):

https://codereview.chromium.org/23072035/diff/1/lss/linux_syscall_support.h#n...
lss/linux_syscall_support.h:2036: : "=r" (res)
On 2013/08/29 20:22:03, Julien Tinnes wrote:
> This should always have been =r, right?

Yes.  Constraining this to rax was unnecessary.

[Mark Seaborn, 2013-08-31 00:05:50]

Committed patchset #1 manually as r23 (presubmit successful).