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

Christian Thalinger christian.thalinger at oracle.com
Thu Sep 19 18:34:11 PDT 2013

On Sep 19, 2013, at 6:06 PM, Lois Foltan <lois.foltan at oracle.com> wrote:

> On 9/19/2013 7:25 PM, Christian Thalinger wrote:
>> On Sep 19, 2013, at 4:22 PM, Lois Foltan <lois.foltan at oracle.com> wrote:
>>> On 9/19/2013 6:27 PM, Christian Thalinger wrote:
>>>> 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 in the case of CAST_TO_OOP, where the type to cast value to is known, an oop.  In the case of CAST_FROM_OOP, it wouldn't work as nicely without having to introduce many different inline methods based on the different types that an oop must be cast to.
>> How about a template method?
> Hi Christian,
> I had to prototype this idea, here's the implementation I came up with
> template <class T> inline oop cast_to_oop(T value) {
>   return (oop)(CHECK_UNHANDLED_OOPS_ONLY((void *))value);
> }
> template <class T> inline T cast_from_oop(oop& o) {
>   return (T)(CHECK_UNHANDLED_OOPS_ONLY((void*))o);
> }
> The cast_from_oop() template method vs. the CAST_FROM_OOP() macro is a wash, in that no extra copy construction was incurred.  The     cast_to_oop() template method vs. the CAST_TO_OOP() macro was not.  There was one extra call to the (void *) conversion operator and an extra copy construction.  I believe this can be attributed to the return of the oop since the temporary oop that was constructed could not be returned by reference since it is a temporary, thus an extra copy construction occurred to return it by value.  Given the extra copy construction, it is better to stick with the macros.

But this is only when CHECK_UNHANDLED_OOPS is on, right?  In a product there can't be a copy construction.  If that's the case I'd say we go with the template method because the tiny overhead in a fastdebug build is negligible.

> Thanks,
> Lois  
>>> Lois
>>>>>  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

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