[llvm-dev] GCC 5 and -Wstrict-aliasing in JSON.h

[Leslie Zhai via llvm-dev]

Hi all,

Thanks for all your response!

It is a good teaching case I can learn from it, and I forgot to paste 
what I mentioned before:

* Almost the same usecase, but they argue that it should be fixed in the 
compiler side https://github.com/mapbox/variant/issues/148

* Reduced testcase punning.cpp is *not* reproduce for gcc 4.9.3 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80593


在 2018年08月14日 18:55, Kim Gräsman 写道:
> On Tue, Aug 14, 2018 at 11:51 AM Andrew Haley <aph at redhat.com> wrote:
WoW! Andrew Haley :)

>> On 08/12/2018 02:19 PM, Kim Gräsman wrote:
>>> I still feel a little uncomfortable, because I think Jonathan makes an
>>> excellent point -- if GCC thinks there's a strict-aliasing violation
>>> (whether the standard agrees or not) and classifies this as undefined
>>> behavior, we might invoke the optimizers wrath. The warning is a nice
>>> hint that this could happen.
>> Indeed.  And I'm not convinced that the pointer cast is necessary anyway:
>> if the type passed in is a union, why not simply take the union member of
>> the appropriate type?
> As it turns out, Union is not a union ¯\_(ツ)_/¯. (I thought it was, up
> until this point.)
>
> It's a template producing a char array suitably aligned for any number
> of mutually exclusive types, much like
> https://en.cppreference.com/w/cpp/types/aligned_union.
>
> I can't tell if that makes the waters less dangerous, but there's an
> invariant for this code that it only reinterpret_casts out Ts from the
> char buffer that it has previously put in there using placement new.
> As far as I can tell, that should be safe. The local construct in the
> as<T>() member function template, something like 'return
> *reinterpret<T *>(buffer);' is generally unsafe. But the calling code
> always maintains the type invariant (only invoking as<T> for the right
> T), so I'd argue this is fine.
>
> Whether all compilers see through this is another question, and I
> trust you that there may be dragons.
>
> Thanks,
> - Kim

-- 
Regards,
Leslie Zhai