On 27/08/20 13:17 +0200, Jakub Jelinek wrote:
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.
Doh, yes, sorry.
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
