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

Zachary Turner via lldb-dev lldb-dev at lists.llvm.org
Fri Aug 26 13:02:10 PDT 2016

It seems to be.  I will also make the copy constructor =delete() to make
sure this cannot happen again.

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> 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> 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.
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
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