FR (L) JDK-7195622 (round 2): CheckUnhandledOops has limited usefulness now
lois.foltan at oracle.com
Wed Sep 25 13:14:36 PDT 2013
On 9/25/2013 3:06 PM, Christian Thalinger wrote:
> This looks good. Thank you for changing it to template methods instead of using macros.
Great, thank you for taking the time to look at this given your week at
> On Sep 23, 2013, at 2:17 PM, Lois Foltan <lois.foltan at oracle.com> wrote:
>> Please review the following updated fix which has removed the CAST_*_OOP macro
>> implementation in favor of two inlined template methods, cast_to_oop() & cast_from_oop().
>> Bug: JDK8 b44 hotspot:src/share/vm/oops/klass.hpp: Error:Initializing const volatile oop& requires ... &
>> CheckUnhandledOops has limited usefulness now bug links at:
>> 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
>> inlined template methods, cast_*_oop(), were introduced in oops/oopsHierarchy.hpp
>> 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
>> // 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.
>> Solaris (12u1 & 12u3 C++ compilers),
>> MacOS (llvm-g++ & clang++ compilers),
>> Linux (g++ v4.4.3 & g++ v4.7.3),
>> 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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