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

Zachary Turner via lldb-dev lldb-dev at lists.llvm.org
Fri Aug 26 11:36:14 PDT 2016

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.

On Fri, Aug 26, 2016 at 11:29 AM Greg Clayton <gclayton at apple.com> wrote:

> > On Aug 26, 2016, at 11:24 AM, Greg Clayton via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> >>
> >> 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????
> 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 &".
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