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  1. JDK
  2. JDK-8199698

Change 8199275 breaks template instantiation for xlC (and potentially other compliers)


    • Type: Bug
    • Status: Closed
    • Priority: P2
    • Resolution: Fixed
    • Affects Version/s: 11
    • Fix Version/s: 11
    • Component/s: hotspot
    • Labels:
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    • Resolved In Build:


      Change "8199275: Fix inclusions of allocation.inline.hpp" replaced the inclusion of "allocation.inline.hpp" in some .hpp files (e.g. constantPool.hpp) by "allocation.hpp".

      "allocation.inline.hpp" contains not only the definition of some inline methods (as the name implies) but also the definition of some template methods (notably the various CHeapObj<>::operator new() versions).

      Template functions are on orthogonal concept with regard to inline functions, but they share on implementation communality: at their call sites, the compiler absolutely needs the corresponding function definition. Otherwise it can either not inline the corresponding function in the case of inline functions or it won't even be able to create the corresponding instantiation in the case of a template function.

      Because of this reason, template functions and methods are defined in their corresponding .inline.hpp files in HotSpot (even if they are not subject to inlining). This is especially true for the before mentioned CHeapObj<>:: new operators, which are explicitly marked as "NOINLINE" in allocation.hpp but defined in allocation.inline.hpp.

      Now every call site of these CHeapOb<>::new() operators which only includes "allocation.hpp" will emit a call to the corresponding instantiation of the CHeapObj<>:: new operator, but wont be able to actually create that instantiation (simply because it doesn't see the corresponding definition in allocation.inline.hpp). On the other side, call sites of a CHeapObj<>:: new operator which include allocation.inline.hpp will instantiate the required version in the current compilation unit (or even inline that method instance if it is not flagged as "NOINLINE").

      If a compiler doesn't honor the "NOINLINE" attribute (or has an empty definition for the NOINLIN macro like xlC), he can potentially inline all the various template instances of CHeapObj<>:: new at all call sites, if their implementation is available. This is exactly what has happened on AIX/xlC before change 8199275 with the effect that the resulting object files contained no single instance of the corresponding new operators.

      After change 8199275, the template definition of the CHeapObj<>:: new operators aren't available any more at all call sites (because the inclusion of allocation.inline.hpp was removed from some other .hpp files which where included transitively before). As a result, the xlC compiler will emit calls to the corresponding instantiations instead of inlining them. But at all other call sites of the corresponding operators, the operator instantiations are still inlined (because xlC does not support "NOINLINE"), so we end up with link errors in libjvm.so because of missing CHeapObj<>::new instances.

      This very problem can easily be solved for xlC, by defining the corresponding "NOINLINE" macro for xlC. Unfortunately it's syntax requirements are a little different for method declarations, compared to the other platforms (it has to be placed AFTER the method declarator instead of BEFORE it). Fortunately, there are no differences for method definitions, so we can safely move the NOINLINE attributes from the method declarations in allocation.hpp to the method definitions in allocation.inline.hpp.

      As a general rule of thumb, we should always make template method definitions available at all call sites, by placing them into corresponding .inline.hpp files and including them appropriately. Otherwise, we might end up without the required instantiations at link time.

      Unfortunately, there's no compile time check to enforce this requirement. But we can misuse the "inline" keyword here, by attributing template functions/methods as "inline". This way, the compiler will warn us, if a template definition isn't available at a specific call site. Of course this trick doesn't work if we specifically want to define template functions/methods which shouldn't be inlined, like in the current case :)




            • Assignee:
              simonis Volker Simonis
              simonis Volker Simonis
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