That seems to me a compiler bug, not: "You can't pass structures with std::atomic" elements in them by value. It would crazy to add a type to the C++ language that you couldn't use everywhere. There doesn't seem to be any official restriction. And anecdotally there's a reference to the problem in gcc for PPC (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62259) but that was considered a bug and fixed.
Not that it matters much, you are stuck with the compiler you're stuck with, bugs and all. Jim > On Aug 26, 2016, at 12:26 PM, Zachary Turner <ztur...@google.com> wrote: > > It could, in theory, it just doesn't. I compiled a quick program using > i686-darwin as the target and the generated assembly makes no attempt to pad > the arguments. > > I'll post some code later > > On Fri, Aug 26, 2016 at 11:42 AM Jim Ingham <jing...@apple.com> wrote: > > > On Aug 26, 2016, at 11:36 AM, Zachary Turner via lldb-dev > > <lldb-dev@lists.llvm.org> wrote: > > > > The thing is, the code is already full of data races. I think the > > std::atomic is actually used incorrectly and is not even doing anything. > > > > That said, consider darwin on 32-bit, where I believe each stack frame is > > 4-byte aligned. I dont' think there's any way the compiler can guarantee > > that a function parameter is 8-byte aligned without allocating from the > > heap, which is obviously impossible for a stack variable. > > Why can't the compiler pad the argument slot on the stack so that the actual > struct lives at a properly aligned location? It's copying the value into the > callee's stack frame, so it can put it wherever it wants. And both caller > and callee sites know the alignment requirements from the function signature, > so they can both figure out where the actual struct lives in the argument > slot based on the alignment of the stack slot. > > Jim > > > > > > On Fri, Aug 26, 2016 at 11:29 AM Greg Clayton <gclay...@apple.com> wrote: > > > > > On Aug 26, 2016, at 11:24 AM, Greg Clayton via lldb-dev > > > <lldb-dev@lists.llvm.org> wrote: > > > > > >> > > >> On Aug 26, 2016, at 10:51 AM, Zachary Turner via lldb-dev > > >> <lldb-dev@lists.llvm.org> wrote: > > >> > > >> I recently updated to Visual Studio 2015 Update 3, which has improved > > >> its diagnostics. As a result of this, LLDB is uncompilable due to a > > >> slew of errors of the following nature: > > >> > > >> D:\src\llvm\tools\lldb\include\lldb/Target/Process.h(3256): error C2719: > > >> 'default_stop_addr': formal parameter with requested alignment of 8 > > >> won't be aligned > > >> > > >> The issue comes down to the fact that lldb::Address contains a > > >> std::atomic<uint64_t>, and is being passed by value pervasively > > >> throughout the codebase. There is no way to guarantee that this value > > >> is 8 byte aligned. This has always been a bug, but until now the > > >> compiler just hasn't been reporting it. > > >> > > >> Someone correct me if I'm wrong, but I believe this is a problem on any > > >> 32-bit platform, and MSVC is just the only one erroring. > > >> > > >> I'm not really sure what to do about this. Passing > > >> std::atomic<uint64>'s by value seems wrong to me. > > >> > > >> Looking at the code, I don't even know why it needs to be atomic. It's > > >> not even being used safely. We'll have a single function write the > > >> value and later read the value, even though it could have been used in > > >> the meantime. Maybe what is really intended is a mutex. Or maybe it > > >> doesn't need to be atomic in the first place. > > >> > > >> Does anyone have a suggestion on what to do about this? I'm currently > > >> blocked on this as I can't compile LLDB. > > > > > > Feel free to #ifdef around the m_offset member of Address and you can > > > have the data race and compile. This seems like a compiler bug to me. If > > > a struct/classe by value argument has alignment requirements, then the > > > compiler should handle this correctly IMHO. Am I wrong???? > > > > Or if this isn't a compiler bug, feel free to modify anything that was > > passing Address by value and make it a "const Address &". > > _______________________________________________ > > lldb-dev mailing list > > lldb-dev@lists.llvm.org > > http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev > _______________________________________________ lldb-dev mailing list lldb-dev@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
