I'm glad to see that clang can also optimize this get_if access; it used to
be unable to do so. MSVC is also unable to optimize this when using
__assume (
https://docs.microsoft.com/en-us/cpp/intrinsics/assume?view=vs-2019 )
(which is the same as Clang's __builtin_assume()
https://clang.llvm.org/docs/LanguageExtensions.html#builtin-assume )
instead of __builtin_unreachable().

In theory, a portable __builtin_unreachable() would be:

[[noreturn]]
inline void unreachable() {} // basically, unconditionally trigger
undefined behavior

In practice, only GCC seems to recognize this.

--Justin Bassett

On Sun, Oct 25, 2020 at 5:59 PM Ryan P. Nicholl via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
> [build] C:\Users\Ryan\rpnx-core\private\sources\all\test3.cpp(119,5): error C3861: '__builtin_unreachable': identifier not found [C:\Users\Ryan\rpnx-core\build\rpnx-core-test3.vcxproj]
>
> Great idea, except that __builtin_unreachable() is a GCC specific
> extension, and is not part of standard C++. (It would be nice to be able to
> do this in a cross platform way though! But that is for another discussion.)
>
> --
> Ryan P. Nicholl
> Tel: (678)-358-7765
> Personal Email: rnicholl_at_[hidden]
> Work Email: ryan.nicholl_at_[hidden]
> Tox:
> 52B3A61CDAF88BCC854F568933F071E3530600CDBA3D5354AADC9AD179575D68021AE959719D
>
>
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Sunday, October 25, 2020 8:50 PM, Barry Revzin <barry.revzin_at_[hidden]>
> wrote:
>
>
>
> On Sun, Oct 25, 2020 at 7:41 PM Ryan P. Nicholl via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
>
>> I decided to implement a new class based on std::variant. I call it
>> "rpnx::derivator", but it's basically "allocating_variant". I tried to make
>> it as similar to std::variant as possible. When looking at this, I noticed
>> something weird about std::variant. There is no "zero overhead" way to get
>> the element contained by the variant, as std::get<I> checks for invalid
>> access and throws an exception if invalid. To solve this issue, I would
>> like to propose std::as, which works the same as std::get, but accessing
>> the wrong type is undefined behavior instead of throwing an exception.
>>
>> --
>> Ryan P. Nicholl
>> Tel: (678)-358-7765
>> Personal Email: rnicholl_at_[hidden]
>> Work Email: ryan.nicholl_at_[hidden]
>> Tox:
>> 52B3A61CDAF88BCC854F568933F071E3530600CDBA3D5354AADC9AD179575D68021AE959719D
>>
>
> You can achieve this by using std::get_if() and marking the nullptr case
> as unreachable. For example:
>
> auto f(std::variant<int, double>& v) -> int* {
> return std::get_if<int>(&v);
> }
>
> auto g(std::variant<int, double>& v) -> int* {
> auto p = std::get_if<int>(&v);
> if (not p) __builtin_unreachable();
> return p;
> }
>
> On -O1, f emits a check but g does not: https://godbolt.org/z/9G9fd5.
>
> Barry
>
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>