Re: [PATCHv2] do not throw in std::make_exception_ptr

[Daniel Krügler <daniel dot kruegler at gmail dot com>]

2016-08-05 6:49 GMT+02:00 Gleb Natapov <gleb@scylladb.com>:
> Instead of throwing an exception allocate its memory and initialize it
> explicitly. Makes std::make_exception_ptr more efficient since no stack
> unwinding is needed.

[..]

> +#ifndef _CXXABI_INIT_EXCEPTION_H
> +#define _CXXABI_INIT_EXCEPTION_H 1
> +
> +#pragma GCC system_header
> +
> +#pragma GCC visibility push(default)
> +
> +#include <stddef.h>
> +#include <bits/c++config.h>
> +
> +#ifndef _GLIBCXX_CDTOR_CALLABI
> +#define _GLIBCXX_CDTOR_CALLABI
> +#define _GLIBCXX_HAVE_CDTOR_CALLABI 0
> +#else
> +#define _GLIBCXX_HAVE_CDTOR_CALLABI 1
> +#endif

>    /// Obtain an exception_ptr pointing to a copy of the supplied object.
>    template<typename _Ex>
> @@ -173,7 +184,16 @@ namespace std
>  #if __cpp_exceptions
>        try
>         {
> -         throw __ex;
> +#if __cpp_rtti && !_GLIBCXX_HAVE_CDTOR_CALLABI
> +          void *__e = __cxxabiv1::__cxa_allocate_exception(sizeof(_Ex));
> +          (void)__cxxabiv1::__cxa_init_primary_exception(__e,
> +                                           const_cast<std::type_info*>(&typeid(__ex)),
> +                                           __exception_ptr::__dest_thunk<_Ex>);
> +          new (__e) _Ex(__ex);
> +          return exception_ptr(__e);
> +#else
> +          throw __ex;
> +#endif

Please take this question was a grain of salt: Is it really correct
that the more efficient approach is used, when

!_GLIBCXX_HAVE_CDTOR_CALLABI

instead of

_GLIBCXX_HAVE_CDTOR_CALLABI

?

To me _GLIBCXX_HAVE_CDTOR_CALLABI sounds like a feature but with that
logic it sounds more like it would be constraint, is that correct?

Thanks,

- Daniel