C++17 std::launder and aliasing

[Jonathan Wakely]

On 21/10/16 18:43 +0200, Jakub Jelinek wrote:
>On Fri, Oct 21, 2016 at 05:29:25PM +0100, Jonathan Wakely wrote:
>> >As said, nothing needed for the middle-end.
>>
>> Thanks, Richi.
>>
>> Moving from gcc@ to libstdc++@ as this is now libstdc++-specific.
>>
>> So we can define std::launder as a no-op for GCC. LLVM apparently does
>> have some optimizations that need to be disabled by doing the laundry,
>> and Clang will be adding a T* __builtin_launder(T*) for that.
>
>Or we could add a typegeneric __builtin_launder builtin that would
>immediately fold to its argument.  Then we can change it any time we figure
>out we need to do something there.

That would work too. I shoulda left this thread on gcc@ :-)

As an aside, __has_builtin is pretty useful (like __has_include and
__has_attribute).