[PATCH] D73245: Depend stddef.h to provide max_align_t for C++11 and provide better fallback in <new>

[Joerg Sonnenberger via Phabricator via llvm-commits]

joerg added a comment.

In D73245#1918287 <https://reviews.llvm.org/D73245#1918287>, @ldionne wrote:

> In D73245#1918148 <https://reviews.llvm.org/D73245#1918148>, @EricWF wrote:
>
> > I've already stated my disapproval of this patch. Libc++ has never and will never provide nor value C++03 conformance.
> >  Moving backwards to C++03 is inconsistent with the libraries general direction.
>
>
> @EricWF makes a point here, we want to move away from C++03.


Removing or breaking C++03 compatibility is actively harmful for that. Speaking as a system integrator, a STL implementation that can't deal in a reasonable way with the existing C++03 code base is useless. It's a nice ivory tower assumption that old code will just disappear if you wish for it enough, but it doesn't reflect well on the reality. Especially given the various ways can and does fail when compiled for C++11 or later. Having to switch between different STL implementations doesn't help anyone in the process of modernizing a code base, especially when interacting with code that needs large semi-intrusive changes to work in C++11 mode. As a most basic issue, it makes it much harder to separate issues with the STL implementation (i.e. depending on implementation-defined behavior) from semantic changes in the core language (i.e. noexcept dtors, auto) and mechanical changes (e.g. UDL)

>> This patch disables tests, which could hide bugs, including serious ABI differences between dialects.
>> 
>> I would like to unbreak compilation on NetBSD. But all that's needed there is to provide our own correct declaration of max_align_t.
>>  I don't see why C++03 conformance is a necessary condition.
> 
> Is there anything that can be done on the NetBSD side to solve this? Basically, just imagine that libc++ doesn't provide a C++03 mode at all -- what would you do then? I think that's the right mindset to solve this specific problem.

Can you both please go back to look at the core of the issue? The current behavior of libc++ is at best an ODR violation on ALL platforms. This is not a problem specific to NetBSD at all, it has just been exposed as a different error on NetBSD. The documented behavior of libc++ is to provide C++03 compatibility as much as possible when it doesn't conflict with C++11 and the compiler (clang) supports the necessary language feature. max_align_t is not provided by clang in C++03 nor is it provided by GCC.  Now there are different options for fixes, but it is very frustrating to me that the primary feedback so far is "Don't use C++03" and therefore not helpful at all.  As I said before, libc++ cares about max_align_t exactly for one purpose: whether overaligned types need special allocators. Now C++03 normally can't depend on the implementation to do something useful in this case, simply because the environment doesn't have the necessary means. So it all boils down to what is the least intrusive and potentially most well testable mechanism. The core of the patch provides one answer for that. If the compiler supports telling us the default alignment for new, use it just like we would in C++11 mode. A constructive feedback would now be an answer to the question of whether we care about libc++ use with compilers that do not provide that macro for C++03 mode. I don't know how useful libc++ in C++03 mode is for any compiler except clang and as I wrote, we document the primary target being clang. As such the primary question is whether we care about compatibility with older versions of clang and modern libc++.  If we don't, the change to `<new>` becomes much simpler as it can just bail out and the corresponding testing becomes easier as well, since it would only have to assert that max_align_t is aligned less strictly than the platform allocator. It would solve the core issue that we provide a max_align_t when we don't know what it should be and actively disagree with the max_align_t we provide for the other dialects.


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https://reviews.llvm.org/D73245