Re: is portable aliasing possible in C++?

[Andrew Haley <aph at redhat dot com>]

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.