[cfe-dev] constexpr bit_cast status?

Erik Pilkington via cfe-dev cfe-dev at lists.llvm.org
Wed Oct 31 15:40:04 PDT 2018



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 <mailto: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 <mailto: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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