[cxx-abi-dev] Transfer modes for parameters and return values

[Richard Smith]

On Fri, Nov 23, 2012 at 12:37 PM, Jason Merrill <jason at redhat.com> wrote:

> On 06/03/2009 04:59 PM, David Vandevoorde wrote:
>
>> In 3.1.1 and 3.1.4, the ABI specifies that "by value" class type
>> parameters and class type return values are passed via the address of a
>> reference if the class type has
>>      (a) a nontrivial destructor, or
>>      (b) a nontrivial copy constructor.
>>
>> Should we now also add to that:
>>      (c) a (nontrivial) move constructor
>> ?
>>
>
> This change doesn't seem to have been applied to the ABI, though G++
> implements it.  What are other compilers doing?


I happened to be looking into this very recently... Clang implements the
ABI as currently specified (along with the corresponding miscompiles).
However, the proposed approach doesn't appear to remove all the
miscompiles, for at least two reasons:

1) A parameter can be passed or a return value constructed by calling a
constructor which is not a copy or move constructor. This can happen when a
templated constructor is selected:

struct A {
  A(const A&) = default; // suppress implicit move constructor, still
trivial
  template<typename T> A(T &&); // chosen for copies from non-const objects
and for moves, not a copy constructor nor a move constructor
};

2) A parameter can be passed or a return value constructed by
copy-list-initialization with no copy or move occurring:

struct B {
  B(int);
  B(const B&) = delete; // deleted, but still trivial
  B(B&&) = delete; // likewise
  B *p = this;
};
B f(B x) {
  return { 1 };
}
void g() {
  f({2});
}

Both of these cases can lead to miscompiles with both g++ 4.7 and Clang
trunk due to the ABI's rules. Both problems can also arise for classes with
no user-declared special members.

The second issue in particular seems unsolvable without a major ABI break
or a core language change (we cannot pass types in registers if they have
any non-trivial constructors).
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