Re: is portable aliasing possible in C++?

[Andy Webber <andy@aligature.com>]

Know of any way to ask Jason Merrill or Richard Biener to weigh in?
They seem to be very knowledgable in this area.

On 9/4/14, Andrew Haley <aph@redhat.com> wrote:
> On 09/04/2014 06:18 PM, Andy Webber wrote:
>> On 9/4/14, Andrew Haley <aph@redhat.com> wrote:
>>> On 09/04/2014 05:11 PM, Andy Webber wrote:
>
> Regrettably,
>>>> Our goal is to avoid bugs caused by strict aliasing in our networking
>>>> libraries.  My question is how to guarantee that we're not violating
>>>> the aliasing rules while also getting the most optimization.  I've
>>>> read through a ton of information about this online and in some gcc
>>>> discussions, but I don't see a consensus.
>>>>
>>>> Memcpy always works, but is dependent on optimization to avoid copies.
>>>> The union of values is guaranteed to work by C++11, but may involve
>>>> copies.
>>>
>>> Is this a real worry?  IME it makes copies when it needs to.
>>>
>>>> Each test works when built with -O3 on gcc-4.8.3, but I would like to
>>>> standardize across compilers and versions.  The optimization
>>>> information generated by -fdump-tree-all is interesting here as it
>>>> shows slightly different optimization for each case though
>>>> reinterpret_cast and placement new generate identical code in the end.
>>>
>>> The "union trick" has always worked with GCC, and is now hallowed by
>>> the standard.  It's also easy to understand.  It generates code as
>>> efficient as all the other ways of doing it, AFAIAA.  It's what we
>>> have always recommended.
>>>
>>> Your test is nice.  I suppose we could argue that this is a missed
>>> optimization:
>>>
>>> union_copy():
>>>         movl    $2, %eax
>>>         cmpw    $2, %ax
>>>         jne     .L13
>>>
>>> I don't know why we only generate code for one of the tests.
>>
>> Thanks for responding. I appreciate any clarity that the gcc devs and
>> standards experts can give here.
>>
>> I'm especially interested in the validity of the placement new
>> approach.   Your recommendation of going through unions causes some
>> difficulty for us in terms of type abstraction. Specifically,
>> receiving network bytes directly into a union with all possible
>> message types present in the union is somewhat less flexible than
>> determining the correct message type and doing a placement new to
>> create essentially a memory overlay.   Is placement new a suitable
>> substitute for __may_alias__ in this specific example?
>
> I regret that the exact legality of placement new in this context is
> beyond me.  I think it's OK as long as you only do it with POD-types, but
> I'd have bounce this off someone like Jason Merrill.
>
> Andrew.
>
>
>