Shenandoah: SBSA::load_at() should save/restore registers when calling SATB barrier

[Roman Kennke]

Is it ok for that particular piece of code to not push/pop rax and rbx?
Otherwise you may just use push_CPU_state() and pop_CPU_state() which
may also fit your bill?

Otherwise seems ok.

Roman


> In SBSA::load_at(), it calls shenandoah_write_barrier_pre() for
> references that need to be kept alive. However, the call has
> side-effects that may mess up registers. Especially, the slow-path
> contains runtime call.
> 
> This problem impacts c2i_entry_barrier(), where upstream uses
> resolve_weak_handle() which keeps the handle alive, while Shenandoah
> uses peek_weak_handle(), which does not keep the handle alive, so it
> bypasses SATB barrier. I think this just hides the problem.
> 
> Shenandoah should just save and restore register around
> shenandoah_write_barrier_pre() call to eliminate register issues, hence,
> it can fallback to upstream c2i_entry_barrier() implementation and
> eliminate the need for peek_weak_handle().
> 
> This fix also fixes the intermittent is_loader_alive() assertion we have
> seen in nightly tests. While this assertion is relative rare on x86_64,
> but it is quite reproducible on x86_32 (because of it has less
> registers(?)). With this fix, I have yet seen once.
> 
> 
> Webrev:
> http://cr.openjdk.java.net/~zgu/shenandoah/load_at_keepalive_fix/webrev.00/
> 
> Test:
>   hotspot_gc_shenandoah (fastdebug and release) x86_64 and x86_32 on Linux
> 
> Thanks,
> 
> -Zhengyu
>