[lldb-dev] Passing std::atomics by value

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Fri Aug 26 11:24:25 PDT 2016

> On Aug 26, 2016, at 10:51 AM, Zachary Turner via lldb-dev <lldb-dev at 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????

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