C++ copy elision and escaping address of return slot

[lhmouse]

在 6/2/21 8:31 PM, Marc Glisse 写道:
> On Wed, 2 Jun 2021, Liu Hao via Gcc-help wrote:
>
>> However I expect there to be a better way to mitigate this: Is it possible to inform GCC to assume
>> that the return slot cannot alias anything?
>
> I assume we will eventually fix
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80740
> it shouldn't be too hard assuming we agree that it is legal.
>
> I am not aware of a way for a user to add restrict on the hidden return pointer.
>

Does that testcase really exist in real codebases? I think it is reasonable to ignore such possibility.

And since you mentioned Clang in that PR: Do you know that Clang assumes an `unsigned short` parameter that is passed via a 32-bit register has its all upper half bits as zeroes [1]? This of course violates psABI, but there has been no problem in practice. Pedantry is sometimes hindrance.


[1] https://gcc.godbolt.org/z/4eoTPdoGx


--
Best regards,
Liu Hao