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

[Eric Fiselier via cfe-dev]

On Tue, Jul 19, 2016 at 9:13 AM, David Blaikie via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
>
> On Mon, Jul 18, 2016 at 3:13 PM Greg Parker via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> On Jul 18, 2016, at 2:08 PM, Richard Smith via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>> On Mon, Jul 18, 2016 at 1:39 PM, Bruno Cardoso Lopes via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Hi,
>>>
>>> C++ static destructors can be problematic in multi-threaded
>>> environment. Some of the issues users often complain about include:
>>> 1. Teardown ordering: crashes when one thread is exiting the process
>>> and calling destructors while another thread is still running and
>>> accessing the destructing variables
>>> 2. Shared code that is compiled both as an application and as a
>>> library. When library mode is chosen, goto (1).
>>> 3. Some projects currently override __cxa_atexit to avoid the behavior
>>> in question.
>>>
>>> To get around that, I propose we add a compiler option (e.g.
>>> -fno-cxx-static-destructors) to allow clang to suppress destructor
>>> registration (currently done via __cxa_atexit, atexit):
>>> https://reviews.llvm.org/D22474
>>>
>>> I'm opening this discussion here on cfe-dev to get some feedback on the
>>> matter
>>>
>>> One can argue that dealing with C++ static destructors in
>>> multi-threaded environment is solely the responsibility of the
>>> developer, however since (AFAIK) we don't have any standard guaranteed
>>> semantic for "global destruction vs. threads", it seems fair to me
>>> that we could give developers some option.
>>
>>
>> They already have options. They can use std::quick_exit, which was added
>> specifically to address this problem, if they don't want destructors to be
>> run at all.
>>
>>
>> std::quick_exit() does not help. The destructor is in a library. The
>> library author has no control over how other code in the process calls
>> exit(). The authors of the app and other libraries are unaware that exit()
>> is dangerous.
>>
>>
>> There are standard techniques to avoid destructors being run for specific
>> objects:
>>
>>   template<typename T> union not_destroyed {
>>     T value;
>>     template<typename ...U> constexpr not_destroyed(U &&...u) :
>> value(std::forward<U>(u)...) {}
>>     ~not_destroyed() {}
>>   };
>>   not_destroyed<std::string> my_str("foo"); // technically has object
>> lifetime issues
>>
>>
>> This is fragile. It's easy to accidentally define a static variable that
>> does not have this template, thereby breaking exit() again.
>>
>
>> -Werror=exit-time-destructors complains about ~not_destroyed(), so it
>> can't help. Adding #pragma diagnostic around every use of not_destroyed
>> would fix that and not be fragile, but it would be awfully ugly.
>>
>
> I think not_destroyed could be written differently so as not to have a
> non-trivial dtor. Make it more like std::optional (wrapping an object in a
> pointer-like API) & just has a byte buffer member and no dtor declared.
> Then it'd be trivially destructible and have no global dtor and be
> -Wexit-time-destructors clean.
>


Clang seems to have no problem removing the call to the empty dtor at -O1.
Using a byte buffer can have other problems, like losing the possibility of
constant initialization.


>
>
>>
>> A post-link test for references to symbol __cxa_atexit might help, but
>> only if there are no intentional static destructors anywhere and only for
>> optimized builds.
>>
>>
>>   std::string &&s = *new std::string("foo");
>>
>>
>> This didn't compile.
>>
>>     test.cxx:12:26: error: rvalue reference to type 'basic_string<[3 *
>> ...]>' cannot bind to lvalue of type 'basic_string<[3 * ...]>'
>>
>>
>> --
>> Greg Parker     gparker at apple.com     Runtime Wrangler
>>
>>
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>
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