[llvm-commits] [PATCH] Fix Alias Bug

[Richard Smith]

On Tue, Jan 8, 2013 at 2:59 PM,  <dag at cray.com> wrote:
> Stephen Canon <scanon at apple.com> writes:
>
>> For C, footnote 95 says the following:
>>
>>
>>     If the member used to read the contents of a union object
>>
>>     is not the same as the member last used to store a value
>>
>>     in the object, the appropriate part of the object representation
>>
>>     of the value is reinterpreted as an object representation in the
>>
>>     new type as described in 6.2.6 (a process sometimes called
>>
>>     ‘‘type punning’’). This might be a trap representation.
>
> I believe the C standard has also changed over the years, to the point
> where there's much confusion about this issue, according to the google.
> :)

This is UB in C++, but is given defined behavior as a GNU extension,
which Clang supports.

>> I am not aware of any similar language for C++, though I don't know
>> the C++ standard nearly as well.  Either way, memcpy( ) is
>> unambiguously correct and should generate good code.
>
> I'm happy to use memcpy if that's what people want.  Honestly, my goal
> was to make gcc shut up, not necessarily guarantee correctness, though
> this is a unit test.  :)

memcpy is the "blessed" C++ approach for this.