[lldb-dev] Passing std::atomics by value
Greg Clayton via lldb-dev
lldb-dev at lists.llvm.org
Fri Aug 26 17:13:20 PDT 2016
There is no need to delete the lldb_private::Address copy constructor. Just stop functions from passing it by value.
> On Aug 26, 2016, at 1:13 PM, Zachary Turner <zturner at google.com> wrote:
> IOW, marking it =delete() is no different than deleting the copy constructor above, but at least if you mark it delete, maybe someone will read the comment above it that explains why it's deleted :)
> On Fri, Aug 26, 2016 at 1:13 PM Zachary Turner <zturner at google.com> wrote:
> 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?
>> 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?
>> > 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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