[PATCH] D142214: [MC] Do not copy MCInstrDescs. NFC.

[Ilya Biryukov via Phabricator via llvm-commits]

ilya-biryukov added a comment.

To describe the problem, the following code breaks in C++20:

  struct WantAggregate {
    WantAggregate(const WantAggregate&) = delete;
    int a;
    int b;
  };
  
  WantAggregate a = {1,2}; // does not compile in C++20.

One way to workaround this would be to force an implicitly-deleted constructor:

  struct NoCopy {
    NoCopy() = default;
    NoCopy(const &) = delete;
  };
  struct WantAggregate {
    int a;
    int b;
    NoCopy delete_copy = {};
  };
  
  WantAggregate a = {1,2}; // works!

However, this approach increases the size of `MCInstrDesc` (the extra member needs to have an address). One way to workaround it is

  [[no_unique_address]] NoCopy delete_copy = {}; // no increase in size, implicitly deleted copy!

Unfortunately, this attribute was added only in C++20. I am trying to figure out a way to make this work in both C++17 and C++20 without the size increase, but still struggling to find a way without a preprocessor.

(I'm assuming the size increase is undesirable, lmk if it's actually not a big deal. Using a member is probably the easiest workaround.)


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D142214