is portable aliasing possible in C++?

[Andy Webber]

On 9/4/14, Andy Webber <andy@aligature.com> wrote:
> 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.
>>
>>
>>
>

Sorry, didn't mean to top post the last reply.

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