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

[Jakub Jelinek]

On Thu, Aug 27, 2020 at 12:06:59PM +0100, Jonathan Wakely wrote:
> 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.

My understanding of
"Padding bits of the To object are unspecified."
is that one shouldn't treat the result of std::bit_cast as if it was e.g.
value initialization followed by member-wise assignment.
But it doesn't say anything about padding bits in the From object.
In the above testcase, T has no padding bits, only S has them.
I think the "Padding bits of the To object are unspecified." should be about:
  T t = { 0, 0 };
  int s = std::bit_cast<T> (std::bit_cast<S> (t)).a;
being UB, that one can't expect the padding bits in S to have a particular
value.

	Jakub