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Subject: Re: [std-proposals] std::as
From: Avi Kivity (avi_at_[hidden])
Date: 2020-11-05 11:09:33


How can the reader of *get_if() tell if the author intended this
optimization, or was just lazy?

Function names should communicate intent. "get_if()" communicates that
you don't know if the variant holds the the type you are asking for or
not. *get_if() communicates that you forgot to check.

My preference would be static_variant_cast<>, following static_cast<>
and static_pointer_cast<>. The latter two communicate that the author
knows there is a safety violation if some other check is not performed,
and confirms they are aware of it, enough to have typed such a long and
ugly name.

On 26/10/2020 03.25, Tony V E via Std-Proposals wrote:
> I don't think you need unreachable.
> *get_if should be enough. Unconditionally dereference the result.
>
> Sent from my BlackBerry portable Babbage Device
> *From: *Justin Bassett via Std-Proposals
> *Sent: *Sunday, October 25, 2020 9:16 PM
> *To: *Std-Proposals
> *Reply To: *std-proposals_at_[hidden]
> *Cc: *Justin Bassett
> *Subject: *Re: [std-proposals] std::as
>
>
> 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
> <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
> <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]
> <mailto: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]
> <mailto:rnicholl_at_[hidden]>
> Work Email: ryan.nicholl_at_[hidden]
> <mailto:ryan.nicholl_at_[hidden]>
> Tox:
> 52B3A61CDAF88BCC854F568933F071E3530600CDBA3D5354AADC9AD179575D68021AE959719D
>
>
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Sunday, October 25, 2020 8:50 PM, Barry Revzin
> <barry.revzin_at_[hidden] <mailto:barry.revzin_at_[hidden]>> wrote:
>
>>
>>
>> On Sun, Oct 25, 2020 at 7:41 PM Ryan P. Nicholl via Std-Proposals
>> <std-proposals_at_[hidden]
>> <mailto: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]
>> <mailto:rnicholl_at_[hidden]>
>> Work Email: ryan.nicholl_at_[hidden]
>> <mailto: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 <https://godbolt.org/z/9G9fd5>.
>>
>> Barry
>
> --
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>
>
>



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