On 07/05/2015 08:52 PM, Steve M. Robbins wrote: > Hi, > > I've heard rumours that GCC 5 is coming :-)
there are even rumours about GCC 6 (defaulting to C++14) ;) > I help maintain several C++ libraries and expect some work is required to get > through this GCC transition. I'd like to understand what I'm doing and do it > right the first time. I'm just an average C++ programmer, not an avid > follower > of GCC nor even of debian lists. So below are lots of questions asked out of > ignorance. Please don't shoot the messenger/questioner. :-) > > Posts here and bug reports reference the wiki page > https://wiki.debian.org/GCC5 . I've read that page and there are a bunch of > notes embedded within it that just confuse me: > > > 1. "The good news is, that GCC 5 now provides a stable libcxx11 ABI, and > stable support for C++11 (GCC version before 5 called this supported > experimental). This required some changes in the libstdc++ ABI, and now > libstdc++6 provides a dual ABI, the classic libcxx98 ABI, and the new > libcxx11 > (FIXME: GCC 5 (<< 5.1.1-20) only provides the classic libcxx98 ABI)." > > What does the FIXME refer to? That the correct statement is "The good new > is, > that GCC 5 (post 5.1.1-20) now provides .."? Or something else? removed, left over when I had no gcc-5 package in experimental. > 2. "libstdc++ doesn't change the soname, provides a dual ABI. Existing C++98 > binary packages will continue to work. Building these packages using g++-5 is > expected to work after build failures are fixed. FIXME: Will they only work > when built with _GLIBCXX_USE_CXX11_ABI set to 0? By default they will use the > new libstdc++ ABI." > > What's the answer to this FIXME? What does the last sentence mean? Does it > mean that code built using g++5 by default uses the new libstdc++ ABI? g++-5 in testing/unstable defaults to the cxx98 ABI (without any choice to use the new ABI), g++-5 in experimental defaults to the new ABI (with the choice to use the old one using -D_GLIBCXX_USE_CXX11_ABI=0). > But if not rebuilt, existing code will use the existing libcxx98 ABI? yes. > The next > paragraph says "GCC 6 is expected to change the c++ standard default ..." -- > but doesn't GCC 5 change the standard default already? No. The compiler still defaults to the c++98 standard. Apparently the naming of the ABI's is confusing, upstream now renamed it to "gcc4-compatible" and "new". Whether this is better ... I don't know. GCC 5 now provides a libstdc++ which conforms to the c++11 standard, and the implementation is supposed to provide a stable ABI. > 3. "Library packages built using -std=c++0x or -std=c++11 may have an ABI > change (unknown yet how many). How to find out about these?" > > What does this mean? Why would they have an ABI change? And how do we find > out? GCC upstream marked the c++11 as experimental in pre GCC 5 versions. There is no guarantee that it worked in the past (however for the majority of transitions in the past, it worked quiet well). I assume you'll only find out when something goes wrong, and you see breakage. For applications, you probably won't notice after the rebuild, for a library you may do a transition, changing the package name. > 4. "If the library exports some symbols having either _cxx11 or B5cxx11 in > the > symbol name, it may be incompatible, if these are symbols which form part of > the public API. To find out if these are part of the public API, you would > need > to build all reverse dependencies and see if any of these new symbols are > referenced." > > Being pedantic: isn't that a signal of ABI (rather than API) change? I didn't call that an API change. I'm just trying to describe where these ABI changes are relevant to trigger a library transition. > More > importantly: supposing my library does export some such symbols. Checking > reverse deps in Debian is of course helpful: if any are referenced, then we > know the interface changed. this is my understanding. > (or does it? what if the reverse-dep uses the > symbol itself internally -- might this show a false-positive?) It's not clear to me what you mean. If a library libfoo has a cxx11 symbol defined, that is used by an app/reverse-dep, then it is not a false positive. Just looking at the undefined symbols in the app without sorting out other cxx11 symbols coming from other libraries (e.g. from libstdc++) might lead to false positives. > But if none > are found, can we conclude the interface has NOT changed? I wouldn't think > so: it may simply be that the code in Debian simply doesn't exercise the > relevant part of the interface. If that is the case: how then can one > convince themself that there really is no change in interface? your decision to make. If you think that this will make an ABI change, maybe ask upstream, and/or change the library name. > Additionally, there are some places where the text confuses me even without > FIXME or README: > > > * "Using different libstdc++ ABIs in the same object or in the same library > is > allowed, as long as you don't try to pass std::list to something expecting > std::cxx11::list or vice versa. We should rebuild everything with g++-5 (once > it is the default). Using g++-4.9 as a fallback won't be possible in many > cases." > > The first sentence doesn't seem connected to the second two sentences. What > does "Using g++-4.9 as a fallback ..." mean? Why isn't it possible? > What cases fail? well, I gave the examples passing list as parameter. PR66145 also is a case of an unwanted incompatibility. > * In "Roadmap for libstdc++", it says "Depending on which libstdc++ ABI is > configured as default ...". > > What is the plan now? Which ABI will be default? see the first sentence of the paragraph. I clarified that the proposal is to do the GCC defaults change and the ABI change in one step. > * Section "libstdc++ c++11 incompatibilities (4.9 and 5)" has a short list of > incompatibilities. Is this the complete list or just examples? well, these are known incompatibilities. If you find more ... > * "Passing std::list to something expecting std::cxx11::list or vice versa." > > Is this an incompatibility just regarding C++11 in 4.9 and 5? What if C++11 > code (built with GCC5) passes a list to an old library built using the > libcxx98 ABI? Wouldn't that also fail? yes, my understanding is that this will fail as well, although already when trying to link that code. Matthias -- To UNSUBSCRIBE, email to debian-devel-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/559bc2f9.7040...@debian.org
