On 09/04/2014 06:18 PM, Andy Webber wrote: > On 9/4/14, Andrew Haley <aph@xxxxxxxxxx> 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.
