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

[Ron Durbin]

I am not an official open source code reviewer, but I did review the code and it looks good.

 

Ron

 

From: Lois Foltan 
Sent: Thursday, September 19, 2013 9:11 AM
To: hotspot-runtime-dev at openjdk.java.net; build-dev at openjdk.java.net
Subject: RFR (L) JDK-7195622: CheckUnhandledOops has limited usefulness now

 

 

Please review the following fix: 

    Webrev: 
    HYPERLINK "http://cr.openjdk.java.net/%7Ehseigel/bug_jdk7195622.0/"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