[cfe-dev] [RFC] Suppress C++ static destructor registration

James Y Knight via cfe-dev cfe-dev at lists.llvm.org
Mon Jul 16 15:47:44 PDT 2018

+1 on adding this functionality.

HOWEVER, I think the example of an object (e.g. the objc runtime table)
which you'd like to not destroy is a rather poor motivating example for the
addition of a compiler flag, and would suggest that you rewrite your
motivation without that example in it.

The reason to add a compiler-flag is to enable compiling all your code in
that way -- e.g. if you know you want to _never_ emit global destructors in
your application.

For the objc-runtime table, using a "template <typename T> class
NoDestructor" template which does a new-into-std::aligned_storage dance
seems like an entirely suitable answer. Or, if that's not good-enough for
some reason, then it'd seem to call for the addition of an attribute to
suppress the destructor on the one global it's attached to.

On Mon, Jul 16, 2018 at 5:55 PM JF Bastien via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi folks,
> I’d like to add a flag in clang, -fno-cxx-static-destructors,  which
> allows developers to demand that no static destructors be emitted. Bruno
> has a sample implementation <https://reviews.llvm.org/D22474>. We’ve discussed
> this previously
> <http://lists.llvm.org/pipermail/cfe-dev/2016-July/050040.html> but I’d
> like to re-open the discussion and make a different case for it because
> we’ve received more requests for such a feature.
> *Why is this desirable?*
> In low-memory circumstances it’s often the case that we know that static
> global destructors are never called. It would be useful to avoid emitting
> them entirely to save memory. We can’t necessarily make the types
> themselves trivially destructible (e.g. a std::map, or a type that’s used
> both as a global and as an automatic variable for which the destructor is
> only meaningful when automatic, or coming from 3rd party library such as
> boost), and using a NeverDestroyed<T> class or global pointer only (std::string&
> foo = *new std::string(“derp");) prevents constexpr and is annoying
> boilerplate (and again, 3rd party code breaks that party).
> This is also useful for some thread-related use cases: we have empirical
> evidence that threads using globals cause crashes if these globals are
> being destroyed.
> Thread-local storage is similarly painful for different reasons. I’m not
> proposing that anything be done yet, but let’s keep it in mind.
> *Developers want this?*
> Yes, we’ve received numerous requests for this. Developers are worried
> about code footprint, and have numerous crash reports they’d like to get
> rid of. Developers tell us they’d really rather not pay for this feature,
> because they don’t want to use it yet are stuck with it (and C++ is a
> “don’t pay for what you don’t use” language).
> Interesting note: developers are used to having no cleanup on termination
> on platforms where applications can get terminated when e.g. they’re sent
> to the background by user action.
> *Concrete example*
> Greg Parker provided this example of thread-related issues in the previous
> discussion:
> The Objective-C runtime has a global table that stores retain counts.
> Pretty much all Objective-C code in the process uses this table. With
> global destructors in place this table is destroyed during exit(). If any
> other thread is still running Objective-C code then it will crash.
> Currently the Objective-C runtime avoids the destructor by initializing
> this table using placement new into an aligned static char buffer.
> I’m assuming that the embedded usecase is obvious enough to everyone to
> not need an example. Let me know if that’s not the case.
> *What about standardization?*
> If this works out I'll discuss standardization options through SG14 (and
> then WG21). The type of developer who’s asked for this are typical SG14
> targets (embedding, gaming, etc). This might fit in with “freestanding” or
> other similar SG14 efforts, but we need experience to guide the proposal.
> Maybe EWG will be interested as well? 🤷‍♂️
> Thanks,
> JF
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