[lldb-dev] Use of function local statics

[Zachary Turner]

Thanks.  I will submit another patch first (which adds more global
statics), and then fix this in a follow-up patch.  It's easier to do it
that way rather than submit the fix first and deal with the merge conflict
that would arise.


On Thu, Aug 21, 2014 at 10:28 AM, Greg Clayton <gclayton at apple.com> wrote:

> We already have global place for intialization:
>
> lldb_private::Initialize ()
>
> This is in lldb.cpp. Feel free to add a HostInfoBase::Initialize() to this
> function. This will solve your problems.
>
> Greg
>
> On Aug 21, 2014, at 10:10 AM, Zachary Turner <zturner at google.com> wrote:
> >
> > BTW, another problem with std::call_once is that, like thread-safe
> function local statics as well, any kind of thread synchronization is
> undesirable if it can be avoided.
> >
> >
> > On Thu, Aug 21, 2014 at 10:09 AM, Zachary Turner <zturner at google.com>
> wrote:
> > std::call_once would solve the problem, but it's also going to be ugly
> putting that all over the place.  Another potential solution is to do
> something like this:
> >
> > // HostInfoBase.h
> > class HostInfoBase
> > {
> > public:
> >     static void Initialize();
> >
> > private:
> >     static HostInfoFields *m_fields;
> > };
> >
> > // HostInfoBase.cpp
> >
> > struct HostInfoFields
> > {
> >     uint32_t m_number_cpus;
> >     std::string m_lldb_shared_library_path;
> >     // etc
> > };
> >
> > HostInfoFields *HostInfoBase::m_fields = nullptr;
> >
> > void HostInfoBase::Initialize()
> > {
> >     m_fields = new HostInfoFields();
> > }
> >
> > // lldb-main.cpp
> > int main(int argc, char **argv)
> > {
> >     HostInfo::Initialize();
> > }
> >
> >
> > Thoughts?
> >
> >
> > On Thu, Aug 21, 2014 at 10:03 AM, Greg Clayton <gclayton at apple.com>
> wrote:
> >
> > > On Aug 20, 2014, at 2:36 PM, Zachary Turner <zturner at google.com>
> wrote:
> > >
> > > As part of my moving code from Host to HostInfo, I moved some
> function-local statics to global class-member statics.  The reason for this
> is that MSVC doesn't support thread-safe function local statics until
> VS2014, which is still only in technology preview, whereas LLVM, clang, and
> by extension LLDB support building as far back as VS2012.
> > >
> > > Greg submitted r216080 to convert these global statics back to
> function-local statics, but this had a bug in it which broke things for all
> platforms, so I reverted it in r216123.  A simple fix would have just been
> to address the bug, but my original transition from function-local statics
> to global statics was intentional due to the fact that any use of them on a
> non-primitive type is undefined behavior on MSVC.
> > >
> > > So, I want to see if people have a strong preference one way or the
> other.  If the issue is just silencing the compiler warning that clang
> gives about global constructors, then we can do that in CMake and/or the
> Xcode project.  On the other hand, I understand that global static
> constructors increase application startup times.  Is this a concern for
> anyone?  If so, I can try to come up with a solution.  I think if we try to
> keep the use of statics to a minimum, and make sure that they are generally
> simple types (e.g std::string, which simply does a malloc), then there
> should be no noticeable performance impact on startup.
> > >
> > > Thoughts?
> >
> > For our build submissions here at Apple we need to keep the number of
> global constructors to a minimum. We need to apply for exceptions for each
> global constructor that is added to a shared library or framework. This is
> the main reason for the change I made. global constructors are fine for
> apps and they get to make that decision, but for shared libraries, they
> should be avoided if possible.
> >
> > I would suggest using std::once for any issues you run into:
> >
> > static std::once_flag g_once_flag;
> > std::call_once(g_once_flag, [](){
> >     // Insert code here to run once in a thread safe way
> > });
> >
> >
> >
> >
> >
>
>
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