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

Mehdi Amini via lldb-dev lldb-dev at lists.llvm.org
Fri Aug 26 13:11:06 PDT 2016

> On Aug 26, 2016, at 1:02 PM, Zachary Turner via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> It seems to be.  I will also make the copy constructor =delete() to make sure this cannot happen again.

Just curious: if a member has a deleted copy-ctor (like std::atomic right?), isn’t the copy constructor automatically deleted?


> I'm still concerned that the std::atomic is unnecessary, but at that point at least it just becomes a performance problem and not a bug.
> On Fri, Aug 26, 2016 at 1:00 PM Greg Clayton <gclayton at apple.com <mailto:gclayton at apple.com>> wrote:
> So after speaking with local experts on the subject, we do indeed have a problem. Please convert all placed where we pass lldb_private::Address by value to pass by "const Address &". Anyone that is modifying the address should make a local copy and work with that.
> Is Address the only class that is causing problems?
> Greg
> > On Aug 26, 2016, at 10:51 AM, Zachary Turner via lldb-dev <lldb-dev at lists.llvm.org <mailto: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.
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