[PATCH] c++: Add __builtin_bit_cast to implement std::bit_cast [PR93121]

[Paul Koning]

> On Jul 18, 2020, at 2:50 PM, Jakub Jelinek via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> Hi!
> 
> The following patch adds __builtin_bit_cast builtin, similarly to
> clang or MSVC which implement std::bit_cast using such an builtin too.
> It checks the various std::bit_cast requirements, when not constexpr
> evaluated acts pretty much like VIEW_CONVERT_EXPR of the source argument
> to the destination type and the hardest part is obviously the constexpr
> evaluation.  I couldn't use the middle-end native_encode_initializer,
> because it needs to handle the C++ CONSTRUCTOR_NO_CLEARING vs. negation of
> that, value initialization of missing members if there are any etc., and
> needs to handle bitfields even if they don't have an integral representative
> (I've left out PDP11 handling of those, couldn't figure out how exactly are
> bitfields laid out there).

It seems to be spelled out in builtins.c function c_readstr.

	paul