[cfe-dev] constexpr bit_cast status?

JF Bastien via cfe-dev cfe-dev at lists.llvm.org
Mon Oct 29 13:09:31 PDT 2018

Richard recently made __builtin_memcpy constexpr in some cases: https://reviews.llvm.org/rL338941 <https://reviews.llvm.org/rL338941>
His patch currently disallows all type-puns.

Erik is looking at implementing a bit_cast intrinsic for clang, 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 <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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