On 9/4/14, Andy Webber <andy@xxxxxxxxxxxxx> 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@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. >> >> >> > 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.
