RFR (L) JDK-7195622: CheckUnhandledOops has limited usefulness now

Christian Thalinger christian.thalinger at oracle.com
Thu Sep 19 15:27:21 PDT 2013

On Sep 19, 2013, at 3:19 PM, Lois Foltan <lois.foltan at oracle.com> wrote:

> On 9/19/2013 6:09 PM, Christian Thalinger wrote:
>> + #define CAST_TO_OOP(value) ((oop)(CHECK_UNHANDLED_OOPS_ONLY((void *))(value)))
>> + #define CAST_FROM_OOP(new_type, value) ((new_type)(CHECK_UNHANDLED_OOPS_ONLY((void *))(value)))
>> Could these two macros also be a method?
> Hi Christian,
> I assume by method you are implying methods within the oop class itself?

Not necessarily.  We already have a couple of inline methods is globalDefinitions.hpp.  Would that work?

>  That would work only in the case of fastdebug builds where an oop is defined as a class.  In non-fastdebug builds, an oop is a (oopDesc *).  The macros provided a way to preserve the existing cast to & from an oop to a numerical type in all builds, even non-fastdebug ones.
> Thanks for the initial review,
> Lois
>> On Sep 19, 2013, at 8:13 AM, Lois Foltan <lois.foltan at oracle.com> wrote:
>>> Please review the following fix:
>>>    Webrev:
>>> http://cr.openjdk.java.net/~hseigel/bug_jdk7195622.0/
>>> Bug: JDK8 b44 hotspot:src/share/vm/oops/klass.hpp: Error:Initializing const volatile oop& requires ... &
>>>        CheckUnhandledOops has limited usefulness now bug links at:
>>> https://bugs.openjdk.java.net/browse/JDK-7180556
>>> https://bugs.openjdk.java.net/browse/JDK-7195622
>>> Summary of fix:
>>> Update the C++ oop structure definition in oopsHierarchy.hpp to solve several problems with the current definition when compiled with various C++ compilers across supported platforms.  These changes initially address the problem reported in JDK-7180556 and continue with additional improvements to allow CHECK_UNHANDLED_OOPS to be defined in all fastdebug builds on all platforms as suggested in JDK-7195622.  Several notes concerning this fix:
>>>            1. A review should start at understanding the changes made to oopsHierarchy.hpp
>>>                    a.  Addition of a non-volatile copy constructor to address compile time errors
>>>                         reported in JDK-7180556 and also currently by g++ compilers on Linux.
>>>                    b.  Addition of member wise assignment operators to handle many instances
>>>                         of [non]volatile to [non]volatile oops within the JVM.
>>>                        Note: Solaris compilers would not allow for the member wise assignment operators
>>>                                   of every flavor of non-volatile to volatile and vice versa.  However, unlike g++ compilers,
>>>                                   Solaris compilers had no issue passing a volatile "this" pointer to a non-volatile
>>>                                   assignment operator.  So the g++ compilers needed these different flavors
>>>                                   of the assignment operator and Solaris did not.
>>>                    d. For similar reasons as 1b, addition of a volatile explicit conversion from oop -> void *.
>>>                        g++ specifically complained when trying to pass a volatile "this" pointer.
>>>                    e.  Removal of the ambiguous behavior of having overloaded  copy constructor and
>>>                         explicit user conversion member functions defined of both integral and void *.
>>>                         All C++ compilers, except Solaris, issue a compile time error concerning this ambiguity.
>>>            2. Change #1e had the consequence of C++ compilers now generating compile time
>>>                errors where the practice has been to cast an oop to and from an integral type such
>>>                as intptr_t.  To allow for this practice to continue, when oop is a structure and not
>>>                a OopDesc *, as is the case when CHECK_UNHANDLED_OOPS is defined, two new
>>>                macros were introduced within globalDefinitions.hpp, CAST_TO_OOP and CAST_FROM_OOP.
>>>            3. Due to the change in #1a & #1b, the oop structure in no longer considered a POD (plain old data)
>>>                by the g++ compilers on Linux and MacOS. This caused several changes to be needed
>>>                throughout the JVM to add an (void *) cast of an oop when invoking print_cr().
>>>            4. Many instances of an assignment to a volatile oop required a "const_cast<oop&>" to
>>>                cast away the volatileness of the lvalue. There is already precedence for this
>>>                type of change within utilities/taskqueue.hpp.  The following comment was in place:
>>>            // g++ complains if the volatile result of the assignment is
>>>            // unused, so we cast the volatile away.  We cannot cast
>>>       directly
>>>            // to void, because gcc treats that as not using the result
>>>       of the
>>>            // assignment.  However, casting to E& means that we trigger an
>>>            // unused-value warning.  So, we cast the E& to void.
>>>            5. The addition of the volatile keyword to the GenericTaskQueue's pop_local() & pop_global()
>>>                member functions was to accommodate the Solaris C++ compiler's complaint about the assignment
>>>                of the volatile elements of the private member data _elems when GenericTaskQueue is instantiated
>>>                with a non-volatile oop.  Line #723 in pop_local().  This was a result of #1b, Solaris' lack of
>>>                allowing for all flavors of volatile/non-volatile member wise assignment operators.
>>>                Per Liden of the GC group did pre-review this specific change with regards to performance
>>>                implications and was not concerned.
>>>            6. In utilities/hashtable.cpp, required CHECK_UNHANDLED_OOPS conditional for the instantiation
>>>                of "template class Hashtable<oop, mtSymbol>".  With CHECK_UHANDLED_OOPS specified for a
>>>                fastdebug build, an unresolved symbol occurred.
>>>                        philli:%  nm --demangle build//linux_amd64_compiler2/fastdebug/libjvm.so | grep Hashtable | grep seed
>>>                                         U Hashtable<oop, (unsigned short)2304>::_seed
>>>                        0000000000848890 t Hashtable<oop, (unsigned short)256>::seed()
>>>                        ...
>>> Making these improvements allows for CHECK_UNHANDLED_OOPS to be defined in all fastdebug builds across the supported platforms.
>>> Builds:
>>> Solaris (12u1 & 12u3 C++ compilers),
>>> MacOS (llvm-g++ & clang++ compilers),
>>> Linux (g++ v4.4.3 & g++ v4.7.3),
>>> VS2010
>>> Tests:
>>> JTREG on MacOS,
>>> vm.quick.testlist on LInux,
>>> nsk.regression.testlist, nsk.stress.testlist on LInux,
>>> JCK vm on Windows
>>> Thank you, Lois

More information about the hotspot-dev mailing list