Request for review(M): 4930919: race condition in MDO creation at back branch locations

[Vladimir Kozlov]

Igor Veresov wrote:
> The problem arises when a thread fails to allocate an MDO (by loosing 
> the race) and so profile_method() returns 0. And then we reload the MDO 
> pointer and see it's not null and has been just set by another thread 
> which makes us decide that the data index value the profile_method() 
> returned is valid.
> 
> The solution is to reuse set_method_data_for_bcp() set up mdp: make 
> profile_method() just to create MDO and leave bcp->di translation for 
> set_method_data_for_bcp().
> 
> Webrev: http://cr.openjdk.java.net/~iveresov/4930919/webrev.00/

Looks good. One suggestion.
Swapping registers in addptr() will allow you to avoid double branch:

     testptr(rax, rax);
!   jcc(Assembler::zero, set_mdp); // set mdp to zero

     // rbx,: method
     // rsi: bcp
     call_VM_leaf(CAST_FROM_FN_PTR(address, InterpreterRuntime::bcp_to_di), rbx, rsi);
     // rax,: mdi

+   // mdo is guaranteed to be non-zero here, we checked for it before the call.
     movptr(rbx, Address(rbx, in_bytes(methodOopDesc::method_data_offset())));
     testptr(rbx, rbx);
     jcc(Assembler::zero, zero_continue);
     addptr(rbx, in_bytes(methodDataOopDesc::data_offset()));
-   addptr(rbx, rax);
+   addptr(rax, rbx);
+   bind(set_mdp);
-   movptr(Address(rbp, frame::interpreter_frame_mdx_offset * wordSize), rbx);
+   movptr(Address(rbp, frame::interpreter_frame_mdx_offset * wordSize), rax);

Vladimir