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

[Jonathan Wakely]

On 27/08/20 12:46 +0200, Jakub Jelinek wrote:
>On Thu, Aug 27, 2020 at 12:06:13PM +0200, Jakub Jelinek via Gcc-patches wrote:
>
>Oops, rewrote the testcase from __builtin_bit_cast to std::bit_cast without
>adjusting the syntax properly.
>Also, let's not use bitfields in there, as clang doesn't support those.
>So, adjusted testcase below.  clang++ rejects all 6 of those, but from what
>you said, I'd expect that u and z should be well defined.
>
>#include <bit>
>
>struct S { short a; int b; };
>struct T { int a, b; };
>
>constexpr int
>foo ()
>{
>  S a = S ();
>  S b = { 0, 0 };
>  S c = a;
>  S d;
>  S e;
>  d = a;
>  e = S ();
>  int u = std::bit_cast<T> (a).a; // Is this well defined due to value initialization of a?

The standard says that padding bits in the bit_cast result are
unspecified, so I don't think they have to be copied even if the
source object was zero-initialized and has all-zero padding bits.