[PATCH] D142446: [MC] Disable copying and moving of MCInstrDescs the C++20 way

[David Blaikie via Phabricator via llvm-commits]

dblaikie added a comment.

In D142446#4079439 <https://reviews.llvm.org/D142446#4079439>, @ilya-biryukov wrote:

> In D142446#4078204 <https://reviews.llvm.org/D142446#4078204>, @dblaikie wrote:
>
>> Alternatively, how bad would it be if aggregate initialization were removed and these objects were initialized with ctor calls? (bad for an -O0 build, no doubt... (how bad?) calling all those ctors to build the tables, but does clang optimize all the ctor calls away to the equivalent of aggregate init when using optimizations?)
>
> The alternative suggestion would be to add some library support, roughly this:
>
>   #include <cstdint>
>   
>   struct NonCopyableNonMovable {
>     NonCopyableNonMovable() = default;
>     NonCopyableNonMovable(const NonCopyableNonMovable&) = delete;
>     NonCopyableNonMovable(NonCopyableNonMovable&&) = delete;
>     NonCopyableNonMovable &operator=(const NonCopyableNonMovable &) = delete;
>     NonCopyableNonMovable &operator=(NonCopyableNonMovable &&) = delete;
>   };
>   
>   #if defined(__has_cpp_attribute) && __has_cpp_attribute(no_unique_address)
>   #define LLVM_NO_UNIQUE_ADDRESS [[no_unique_address]]
>   #elif defined(__has_cpp_attribute) && __has_cpp_attribute(msvc::no_unique_address)
>   #define LLVM_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
>   #else
>   #error "WTF"
>   #define LLVM_NO_UNIQUE_ADDRESS
>   #endif
>   
>   #if __cplusplus >= 202002L
>   // Put as the last member in the aggregate struct.
>   #define LLVM_DISABLE_COPY_AND_MOVE(X)\
>       LLVM_NO_UNIQUE_ADDRESS NonCopyableNonMovable DisableCopy##X;
>   #else
>   #define LLVM_DISABLE_COPY_AND_MOVE(X)\
>       X(X const&) = delete;\
>       X(X&&) = delete;\
>       X& operator=(X const&) = delete;\
>       X& operator=(X&&) = delete;
>   #endif
>
> And use it like this:
>
>   struct Y {
>       uint64_t a;
>       uint64_t b;
>   
>       LLVM_DISABLE_COPY_AND_MOVE(Y);
>   };
>   auto y = Y{1, 2};
>   static_assert(sizeof(Y) == 16);
>
> Seem to work in both C++17 and C++20, except in the latest MSVC (looks like a bug, but I'm not entirely certain):
> https://gcc.godbolt.org/z/8nsooKz8c
> I suspect this could be worked around, but I did not dig any further.
>
> The question is: how useful is it and does it carry its weight?

Sounds OK to me, but yeah, might be overkill.

@foad any sense of how important this is? whether it's worth the above compelxity?


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https://reviews.llvm.org/D142446