Author: Chuanqi Xu Date: 2024-07-10T11:23:14+08:00 New Revision: 62a7562b828ee9df6a41b6ce3dfda7c5511816a6
URL: https://github.com/llvm/llvm-project/commit/62a7562b828ee9df6a41b6ce3dfda7c5511816a6 DIFF: https://github.com/llvm/llvm-project/commit/62a7562b828ee9df6a41b6ce3dfda7c5511816a6.diff LOG: [docs] [C++20] [Modules] Add a workaround document for missing vtables Due to https://github.com/llvm/llvm-project/pull/75912 is reverted and https://github.com/llvm/llvm-project/issues/70585 is reopened. It looks riskful to fix the issue correctly before 19. So we provide a workaround here to help people in this trouble as much as possible. Added: Modified: clang/docs/StandardCPlusPlusModules.rst Removed: ################################################################################ diff --git a/clang/docs/StandardCPlusPlusModules.rst b/clang/docs/StandardCPlusPlusModules.rst index 1c3c4d319c0e1..cf0528e75e7f2 100644 --- a/clang/docs/StandardCPlusPlusModules.rst +++ b/clang/docs/StandardCPlusPlusModules.rst @@ -1092,6 +1092,74 @@ A high-level overview of support for standards features, including modules, can be found on the `C++ Feature Status <https://clang.llvm.org/cxx_status.html>`_ page. +Missing VTables for classes attached to modules +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Now the compiler may miss emitting the definition of vtables +for classes attached to modules, if the definition of the class +doesn't contain any key function in that module units +(The key function is the first non-pure virtual function that is +not inline at the point of class definition.) + +(Note: technically, the key function is not a thing for modules. +We use the concept here for convinient.) + +For example, + +.. code-block:: c++ + + // layer1.cppm + export module foo:layer1; + struct Fruit { + virtual ~Fruit() = default; + virtual void eval() = 0; + }; + struct Banana : public Fruit { + Banana() {} + void eval() override; + }; + + // layer2.cppm + export module foo:layer2; + import :layer1; + export void layer2_fun() { + Banana *b = new Banana(); + b->eval(); + } + void Banana::eval() { + } + +For the above example, we can't find the definition for the vtable of +class ``Banana`` in any object files. + +The expected behavior is, for dynamic classes attached to named modules, +the vtable should always be emitted to the module units the class attaches +to. + +To workaround the problem, users can add the key function manually in the +corresponding module units. e.g., + +.. code-block:: c++ + + // layer1.cppm + export module foo:layer1; + struct Fruit { + virtual ~Fruit() = default; + virtual void eval() = 0; + }; + struct Banana : public Fruit { + // Hack a key function to hint the compiler to emit the virtual table. + virtual void anchor(); + + Banana() {} + void eval() override; + }; + + void Banana::anchor() {} + +This is tracked by +`#70585 <https://github.com/llvm/llvm-project/issues/70585>`_. + Including headers after import is not well-supported ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
