[llvm-dev] Placement new and TBAA

[Daniel Berlin via llvm-dev]

What is the purpose of the union there?
I ask because pretty much no compiler will respecting the unioning without
visible accesses  in all cases, because it would ruin most optimization[1]

But i'm also not sure it's required in this testcase to make your testcase
fail.

In practice, handling placement new properly in gcc required the equivalent
of a new intrinsic (in gcc, it required adding CHANGE_DYNAMIC_TYPE_EXPR).





[1]  For example, you can pretty much get every compiler out there to
generate "wrong" code using unions and strict aliasing,  see the last
discussion we had about this on the mailing list (subject line had "wrong
results with union and strict-aliasing").





On Thu, Nov 24, 2016 at 2:40 PM, Hubert Tong via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I've heard from a few people that placement new is going to be handled by
> LLVM using the llvm.invariant.group.barrier intrinsic; however, the
> documentation for that intrinsic seems to indicate that it deals with only
> invariant.group metadata and not TBAA metadata.
>
> I would like to understand how the solution would work in the context of
> https://llvm.org/bugs/show_bug.cgi?format=multiple&id=28767.
>
> -- HT
>
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