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

[Zachary Turner via lldb-dev]

I think so.  But in this case lldb::Address explicitly supplied a copy
constructor that looked like this:

    Address (const Address& rhs) :
        m_section_wp (rhs.m_section_wp),
        m_offset(rhs.m_offset.load())   // this is the std::atomic<>
    {
    }

circumventing the problem.

On Fri, Aug 26, 2016 at 1:11 PM Mehdi Amini <mehdi.amini at apple.com> wrote:

> 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?
>
> —
> Mehdi
>
>
>
> 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.
>> > _______________________________________________
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>>
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