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@xxxxxxxxxx> wrote: > 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. > > >
