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

[Zachary Turner via lldb-dev]

It could, in theory, it just doesn't. I compiled a quick program using
i686-darwin as the target and the generated assembly makes no attempt to
pad the arguments.

I'll post some code later

On Fri, Aug 26, 2016 at 11:42 AM Jim Ingham <jingham at apple.com> wrote:

>
> > On Aug 26, 2016, at 11:36 AM, Zachary Turner via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> >
> > 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.
>
> Why can't the compiler pad the argument slot on the stack so that the
> actual struct lives at a properly aligned location?  It's copying the value
> into the callee's stack frame, so it can put it wherever it wants.  And
> both caller and callee sites know the alignment requirements from the
> function signature, so they can both figure out where the actual struct
> lives in the argument slot based on the alignment of the stack slot.
>
> Jim
>
>
> >
> > 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 &".
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/lldb-dev/attachments/20160826/5d6036da/attachment.html>