[PATCH] D92361: [trivial-abi] Support types without a copy or move constructor.

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D92361#2433775 <https://reviews.llvm.org/D92361#2433775>, @Quuxplusone wrote:

> In D92361#2433190 <https://reviews.llvm.org/D92361#2433190>, @rjmccall wrote:
>
>> There is no such thing as an object "teleporting" in C++.  Objects are always observed in memory with a specific address.  When an argument is passed in registers, its address can be observed to be different on both sides, and that is not permitted; there must be some operation that creates the object at the new address and destroys it at the old.
>
> That's where you're wrong (about C++). You might be right about C or Objective-C, I don't know. In C++, that "teleporting" happens //without// any call to a special member — there is no move happening, and no destroy happening. You can actually observe this: https://godbolt.org/z/zojooc The object is simply "bitwise-teleported" from one place to another. Standard C++17 says that this is a "guaranteed-copy-elision" context; there is indeed only one C++ "object" here. It just happens to blit around in memory //beyond// what the C++ code is doing to it.

I think perhaps we are talking past each other and have reached the limits of what this sub-thread can hope to achieve.

John.


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