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ItaniumLikeC++ABI.md

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This document describes the "Itanium-like" C++ ABI for WebAssembly. As mentioned in README, it's not the only possible C++ ABI or even the only possible Itanium-derived C++ ABI for WebAssembly. It is the ABI that the clang/LLVM WebAssembly backend is currently using, and any other C++ compiler wishing to be ABI-compatible with it.

Most details follow the base Itanium C++ ABI.

The following is a brief summary of deviations from this base:

  • The least-significant bit of the adj field of a member-function pointer is used to indicate whether the indicated function is virtual.
  • Member functions are not specially aligned.
  • Constructors and destructors return this.
  • Guard variables are 32-bit on wasm32.
  • Unused bits of guard variables are reserved.
  • Inline functions are never key functions.
  • C++11 POD rules are used to determine tail padding.

The following are ideas for additional deviations that are being considered:

  • The Itanium C++ name mangling rules have special-case abbreviations for std::string, std::allocator, and a few others, however libc++ doesn’t get to take advantage of them because it uses a versioned namespace. It may be useful to add new special-cases to cover libc++’s mangled names for std::string et al.
  • There’s an interesting idea for a vtable optimization described here that’s worth thinking about.
  • Alternatively, the design of vtables may radically change to take better advantage of WebAssembly’s function table mechanisms.
  • Trivially copyable classes may be passed by value rather than by pointer. This would cover a lot of common C++ classes such as std::pair.

This is just an initial sketch of the kind of content we plan to have here. Obviously it would be desirable to go into a lot more detail.