[all-commits] [llvm/llvm-project] 61bd19: [libc++] Explicitly enumerate std::string external...

[Eric via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 61bd19206f61ace4b007838a2ff8884a13ec0374
      https://github.com/llvm/llvm-project/commit/61bd19206f61ace4b007838a2ff8884a13ec0374
  Author: Eric Fiselier <eric at efcs.ca>
  Date:   2020-01-09 (Thu, 09 Jan 2020)

  Changed paths:
    M libcxx/include/__config
    M libcxx/include/__string
    M libcxx/include/string
    M libcxx/src/string.cpp

  Log Message:
  -----------
  [libc++] Explicitly enumerate std::string external instantiations.

 The external instantiation of std::string is a problem for libc++.
    Additions and removals of inline functions in string can cause ABI
    breakages, including introducing new symbols.

    This patch aims to:
      (1) Make clear which functions are explicitly instatiated.
      (2) Prevent new functions from being accidentally instantiated.
      (3) Allow a migration path for adding or removing functions from the
      explicit instantiation over time.

    Although this new formulation is uglier, it is preferable from a
    maintainability and readability standpoint because it explicitly
    enumerates the functions we've chosen to expose in our ABI. Changing
    this list is non-trivial and requires thought and planning.

    (3) is achieved by making it possible to control the extern template declaration
    separately from it's definition. Meaning we could add a new definition to
    the dylib, wait for it to roll out, then add the extern template
    declaration to the header. Similarly, we could remove existing extern
    template declarations while still keeping the definition to prevent ABI
    breakages.