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

[Jonathan Wakely]

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