[cfe-dev] constexpr bit_cast status?

[JF Bastien via cfe-dev]

It seems simpler to map it directly to the existing IR construct (bitcast) instead of the odd IR memcpy. But 🤷‍♂️ 

> On Oct 31, 2018, at 3:40 PM, Erik Pilkington <erik.pilkington at gmail.com> wrote:
> 
> 
> 
>> On 10/31/18 3:23 PM, Richard Smith via cfe-dev wrote:
>>> On Mon, 29 Oct 2018 at 13:09, JF Bastien via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>>> Richard recently made __builtin_memcpy constexpr in some cases: https://reviews.llvm.org/rL338941
>>> His patch currently disallows all type-puns.
>>> 
>>> Erik is looking at implementing a bit_cast intrinsic for clang,
>> 
>> Is there a reason to make it a new builtin (which I think is what you're suggesting) rather than extending the existing __builtin_memcpy support to cover all the bit_cast cases? (Eg, is the idea to make it exactly cover the cases that std::bit_cast supports?) If we want both, it'd be good to at least make the two share code.
> 
> No real reason I guess, I suppose that approach would lead to a lot less boilerplate, so it probably makes more sense. I'll have a chance to really dig into this in a week or two, so hopefully I'll can put a patch up soon.
> 
>>  
>>> at which point libc++ can use it. ADT won’t be able to do so until we migrate to C++20 (and toolchains support it).
>>> 
>>> We also want to reach out to the RedHat maintainers of libstdc++ to make sure we don’t adopt a weirdly different builtin.
>>> 
>>> 
>>> 
>>>> On Oct 29, 2018, at 12:56 PM, will wray via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>>>> 
>>>> std::bit_cast was added to the C++ 20 draft this summer:
>>>> http://eel.is/c++draft/bit.cast
>>>> 
>>>> In September JFB added a header with implementation (in llvm namespace):
>>>> llvm/include/llvm/ADT/bit.h
>>>> 
>>>>   // This file implements the C++20 <bit> header.
>>>>   ...
>>>>   // - It isn't constexpr because that requires compiler support.
>>>> 
>>>> What compiler support is it waiting on? What's the status?
>>>> 
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